How to undo send in gmail

1-go to google labs and then Gmail labs  or
 click on the green flask in your Gmail as in image to go directly to the gmail labs

2-and then scroll down page to the feature 
3-Now enable this feature.After doing all this, You enabled this feature to your gmail account.send an email and now you will get undo option after sending any email.

Source: http://tricksndtricks.blogspot.com/2011/03/