<?xml version="1.0" encoding="us-ascii"?><InstanceReport xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"><Version>2.4.0.8</Version><ReportLongName>108 - Disclosure - Common stock</ReportLongName><DisplayLabelColumn>true</DisplayLabelColumn><ShowElementNames>false</ShowElementNames><RoundingOption /><HasEmbeddedReports>false</HasEmbeddedReports><Columns><Column FlagID="0"><Id>1</Id><IsAbstractGroupTitle>false</IsAbstractGroupTitle><LabelSeparator>

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</LabelSeparator><Level>4</Level><ElementName>us-gaap_StockholdersEquityNoteDisclosureTextBlock</ElementName><ElementPrefix>us-gaap_</ElementPrefix><IsBaseElement>true</IsBaseElement><BalanceType>na</BalanceType><PeriodType>duration</PeriodType><IsReportTitle>false</IsReportTitle><IsSegmentTitle>false</IsSegmentTitle><IsCalendarTitle>false</IsCalendarTitle><IsEquityPrevioslyReportedAsRow>false</IsEquityPrevioslyReportedAsRow><IsEquityAdjustmentRow>false</IsEquityAdjustmentRow><IsBeginningBalance>false</IsBeginningBalance><IsEndingBalance>false</IsEndingBalance><IsReverseSign>false</IsReverseSign><FootnoteIndexer /><Cells><Cell FlagID="0" ContextID="cx_01_February_2013_TO_31_July_2013" UnitID=""><Id>1</Id><IsNumeric>false</IsNumeric><IsRatio>false</IsRatio><DisplayZeroAsNone>false</DisplayZeroAsNone><NumericAmount>0</NumericAmount><RoundedNumericAmount>0</RoundedNumericAmount><NonNumbericText>&lt;p align="justify" style="font-family: times new roman,times,serif; font-size: 10pt;"&gt;
      &lt;b&gt;NOTE 3 &amp;#8211; Common stock&lt;/b&gt;
    &lt;/p&gt;
    &lt;p align="justify" style="font-family: times new roman,times,serif; font-size: 10pt;"&gt;Our common shares are all of the same class, are voting and entitle stockholders to receive dividends as defined. Upon liquidation or wind-up, stockholders are entitled to participate equally with respect to any distribution of net assets or any dividends that may be declared.&lt;/p&gt;
    &lt;p align="justify" style="font-family: times new roman,times,serif; font-size: 10pt;"&gt;
      In February, March and April, 2013, we issued
      22,874,405
      shares for gross proceeds of $200,000
      related to the investment agreement with Deer Valley Management, LLC.
    &lt;/p&gt;
    &lt;p align="justify" style="font-family: times new roman,times,serif; font-size: 10pt;"&gt;
      In February, 2013, we sold
      3,448,276
      units to one investor for gross proceeds of $40,000. Each unit consisted of one common share of our company and one non-transferable share purchase warrant. Each share purchase warrant entitles the investor to purchase one additional common share of our company at a price of $0.0162
      until February 7, 2016.
    &lt;/p&gt;
    &lt;p align="justify" style="font-family: times new roman,times,serif; font-size: 10pt;"&gt;
      In February, 2013, we issued
      1,526,718
      units to one investor in exchange for gross proceeds of $20,000. Each unit consisted of one common share of our company and one non-transferable share purchase warrant. Each share purchase warrant entitles the investor to purchase one additional common share of our company at a price of $0.0183
      until February 15, 2016.
    &lt;/p&gt;
    &lt;p align="justify" style="font-family: times new roman,times,serif; font-size: 10pt;"&gt;
      In April, 2013, one investor exercised
      3,033,618
      of the May 2007 common stock purchase warrants using the cashless exercise provision. We issued
      2,500,000
      shares of common stock and cancelled
      533,618
      common stock purchase warrants pursuant to the cashless exercise provision. No cash proceeds were received.
    &lt;/p&gt;
    &lt;p align="justify" style="font-family: times new roman,times,serif; font-size: 10pt;"&gt;
      In May, June and July, 2013, we issued
      31,270,958
      shares for gross proceeds of $255,000
      related to the investment agreement with Deer Valley Management, LLC. As of July 31, 2013, we had not yet received payment for one transaction valued at $25,000. Subsequent to July 31, 2013, we received $5,000
      for this transaction and expect to receive the remaining $20,000
      over a period of several months. In August 2013, we decided to terminate the investment agreement with Deer Valley Management, LLC due to their violation of the payment terms pursuant to the investment agreement. No further shares issuances to Deer Valley Management, LLC are expected to occur.
    &lt;/p&gt;
    &lt;p align="justify" style="font-family: times new roman,times,serif; font-size: 10pt;"&gt;
      In May, June and July, 2013, we sold
      18,001,184
      units to five investors for gross proceeds of $182,043. Each unit consisted of one common share of our company and one non-transferable share purchase warrant. The share purchase warrants entitle the investors to purchase one additional common share of our company at prices ranging between of $0.0116
      and $0.0173
      until July 30, 2016.
    &lt;/p&gt;
    &lt;p align="justify" style="font-family: times new roman,times,serif; font-size: 10pt;"&gt;
      In June, 2013, one investor exercised
      4,263,989
      of the May 2007 common stock purchase warrants using the cashless exercise provision. We issued
      3,587,165
      shares of common stock and cancelled
      678,824
      common stock purchase warrants pursuant to the cashless exercise provision. No cash proceeds were received.
    &lt;/p&gt;
        &lt;p align="justify" style="font-family: times new roman,times,serif; font-size: 10pt;"&gt;
      During the six months ended July 31, 2013 there were no stock options granted. At July 31, 2013 there were
      903,500
      non-qualified stock options outstanding with a weighted average exercise price of $ $1.429
      per option; of those options
      872,250
      are exercisable. At July 31, 2013 there were
      90,635,375
      incentive stock options outstanding with a weighted average exercise price of $ $0.047
      per option; of those options
      88,072,875
      are exercisable with a weighted average exercise price of $0.048.
    &lt;/p&gt;
    &lt;p align="justify" style="font-family: times new roman,times,serif; font-size: 10pt;"&gt;
      During the three and six months ended July 31, 2013 we recognized $12,843
      and $47,828, respectively, of compensation expense related to incentive and non-qualified stock options previously granted to officers, employees and consultants.
    &lt;/p&gt;
    &lt;p align="justify" style="font-family: times new roman,times,serif; font-size: 10pt;"&gt;
      As of July 31, 2013, there were
      110,641,700
      whole share purchase warrants outstanding and
      110,610,450
      exercisable. The warrants have a weighted average remaining life of
      1.30
      years and a weighted average exercise price of $$0.052
      per whole warrant for one common share. Whole share purchase warrants outstanding at July 31, 2013 are as follows:
    &lt;/p&gt;
    &lt;div align="center"&gt;
                  &lt;table border="0" cellpadding="0" cellspacing="0" style="border-color: black; border-collapse: collapse; font-size: 10pt; font-family: times new roman,times,serif;" width="90%"&gt;
              &lt;tr valign="top"&gt;
                &lt;td align="left"&gt;&amp;#160;&lt;/td&gt;
                &lt;td align="left" width="1%"&gt;&amp;#160;&lt;/td&gt;
                &lt;td align="center" nowrap="nowrap" width="12%"&gt;Number of whole share&lt;/td&gt;
                &lt;td align="center" nowrap="nowrap" width="2%"&gt;&amp;#160;&lt;/td&gt;
                &lt;td align="center" nowrap="nowrap" width="1%"&gt;&amp;#160;&lt;/td&gt;
                &lt;td align="center" nowrap="nowrap" width="12%"&gt;Weighted average exercise&lt;/td&gt;
                &lt;td align="left" width="2%"&gt;&amp;#160;&lt;/td&gt;
              &lt;/tr&gt;
              &lt;tr valign="top"&gt;
                &lt;td align="left"&gt;&amp;#160;&lt;/td&gt;
                &lt;td align="left" style="BORDER-BOTTOM: #000000 1px solid" width="1%"&gt;&amp;#160;&lt;/td&gt;
                &lt;td align="center" nowrap="nowrap" style="BORDER-BOTTOM: #000000 1px solid" width="12%"&gt;purchase warrants&lt;/td&gt;
                &lt;td align="center" nowrap="nowrap" width="2%"&gt;&amp;#160;&lt;/td&gt;
                &lt;td align="center" nowrap="nowrap" style="BORDER-BOTTOM: #000000 1px solid" width="1%"&gt;&amp;#160;&lt;/td&gt;
                &lt;td align="center" nowrap="nowrap" style="BORDER-BOTTOM: #000000 1px solid" width="12%"&gt;price per share&lt;/td&gt;
                &lt;td align="left" style="BORDER-BOTTOM: #000000 1px solid" width="2%"&gt;&amp;#160;&lt;/td&gt;
              &lt;/tr&gt;
              &lt;tr valign="top"&gt;
                &lt;td align="left" bgcolor="#e6efff"&gt;Outstanding, January 31, 2013&lt;/td&gt;
                &lt;td align="left" bgcolor="#e6efff" width="1%"&gt;&amp;#160;&lt;/td&gt;
                &lt;td align="right" bgcolor="#e6efff" width="12%"&gt;
                  94,963,129
                &lt;/td&gt;
                &lt;td align="left" bgcolor="#e6efff" width="2%"&gt;&amp;#160;&lt;/td&gt;
                &lt;td align="left" bgcolor="#e6efff" width="1%"&gt;$&lt;/td&gt;
                &lt;td align="right" bgcolor="#e6efff" width="12%"&gt;
                  0.058
                &lt;/td&gt;
                &lt;td align="left" bgcolor="#e6efff" width="2%"&gt;&amp;#160;&lt;/td&gt;
              &lt;/tr&gt;
              &lt;tr valign="top"&gt;
                &lt;td align="left"&gt;Issued&lt;/td&gt;
                &lt;td align="left" width="1%"&gt;&amp;#160;&lt;/td&gt;
                &lt;td align="right" width="12%"&gt;
                  22,976,178
                &lt;/td&gt;
                &lt;td align="left" width="2%"&gt;&amp;#160;&lt;/td&gt;
                &lt;td align="left" width="1%"&gt;&amp;#160;&lt;/td&gt;
                &lt;td align="right" width="12%"&gt;
                  0.015
                &lt;/td&gt;
                &lt;td align="left" width="2%"&gt;&amp;#160;&lt;/td&gt;
              &lt;/tr&gt;
              &lt;tr valign="top"&gt;
                &lt;td align="left" bgcolor="#e6efff"&gt;Exercised&lt;/td&gt;
                &lt;td align="left" bgcolor="#e6efff" style="BORDER-BOTTOM: #000000 1px solid" width="1%"&gt;&amp;#160;&lt;/td&gt;
                &lt;td align="right" bgcolor="#e6efff" style="BORDER-BOTTOM: #000000 1px solid" width="12%"&gt;
                  (7,297,607
                &lt;/td&gt;
                &lt;td align="left" bgcolor="#e6efff" width="2%"&gt;)&lt;/td&gt;
                &lt;td align="left" bgcolor="#e6efff" style="BORDER-BOTTOM: #000000 1px solid" width="1%"&gt;&amp;#160;&lt;/td&gt;
                &lt;td align="right" bgcolor="#e6efff" style="BORDER-BOTTOM: #000000 1px solid" width="12%"&gt;
                  0.002
                &lt;/td&gt;
                &lt;td align="left" bgcolor="#e6efff" style="BORDER-BOTTOM: #000000 1px solid" width="2%"&gt;&amp;#160;&lt;/td&gt;
              &lt;/tr&gt;
              &lt;tr&gt;
                &lt;td&gt;&amp;#160;&lt;/td&gt;
                &lt;td width="1%"&gt;&amp;#160;&lt;/td&gt;
                &lt;td width="12%"&gt;&amp;#160;&lt;/td&gt;
                &lt;td width="2%"&gt;&amp;#160;&lt;/td&gt;
                &lt;td width="1%"&gt;&amp;#160;&lt;/td&gt;
                &lt;td width="12%"&gt;&amp;#160;&lt;/td&gt;
                &lt;td width="2%"&gt;&amp;#160;&lt;/td&gt;
              &lt;/tr&gt;
              &lt;tr valign="top"&gt;
                &lt;td align="left" bgcolor="#e6efff"&gt;Outstanding, July 31, 2013&lt;/td&gt;
                &lt;td align="left" bgcolor="#e6efff" style="BORDER-BOTTOM: #000000 1px solid" width="1%"&gt;&amp;#160;&lt;/td&gt;
                &lt;td align="right" bgcolor="#e6efff" style="BORDER-BOTTOM: #000000 1px solid" width="12%"&gt;
                  110,641,700
                &lt;/td&gt;
                &lt;td align="left" bgcolor="#e6efff" width="2%"&gt;&amp;#160;&lt;/td&gt;
                &lt;td align="left" bgcolor="#e6efff" style="BORDER-BOTTOM: #000000 1px solid" width="1%"&gt;$&lt;/td&gt;
                &lt;td align="right" bgcolor="#e6efff" style="BORDER-BOTTOM: #000000 1px solid" width="12%"&gt;
                  0.052
                &lt;/td&gt;
                &lt;td align="left" bgcolor="#e6efff" style="BORDER-BOTTOM: #000000 1px solid" width="2%"&gt;&amp;#160;&lt;/td&gt;
              &lt;/tr&gt;
              &lt;tr valign="top"&gt;
                &lt;td align="left"&gt;Exercisable, July 31, 2013&lt;/td&gt;
                &lt;td align="left" style="BORDER-BOTTOM: #000000 1px solid" width="1%"&gt;&amp;#160;&lt;/td&gt;
                &lt;td align="right" style="BORDER-BOTTOM: #000000 1px solid" width="12%"&gt;
                  110,610,450
                &lt;/td&gt;
                &lt;td align="left" width="2%"&gt;&amp;#160;&lt;/td&gt;
                &lt;td align="left" style="BORDER-BOTTOM: #000000 1px solid" width="1%"&gt;$&lt;/td&gt;
                &lt;td align="right" style="BORDER-BOTTOM: #000000 1px solid" width="12%"&gt;
                  0.052
                &lt;/td&gt;
                &lt;td align="left" style="BORDER-BOTTOM: #000000 1px solid" width="2%"&gt;&amp;#160;&lt;/td&gt;
              &lt;/tr&gt;
          &lt;/table&gt;
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