Gmail's "Undo Send" function

For those of you who use Gmail:

The toolbox button is in the upper, right corner.  It is highlighted with a blue line in this screen shot and the icon is of a six-toothed gear.  Click on the drop-down (the upside down triangle) and then "Settings"

Undo Send is under the "General" tab.

It can be set for 5, 10, 20 or 30 seconds or it can be not  enabled.