Google's Gmail Labs now offers yet another way not to send a message.First there was Email Addict, a Gmail Labs experiment designed to force obsessive Gmail users to take a break after 15 minutes and to block access to instant messages.

Then there was Mail Goggles, which requires that Gmail users answer simple math problems before allowing them to send messages, in order to prevent e-mailing while intoxicated.

Gmail Labs also includes a Forgotten Attachment Detector, which will prevent messages that mention an attachment but don't actually include one from being sent.

And on Thursday, Google added Undo Send, a Gmail Labs experiment that prevents the transmission of an e-mail message for five seconds, to allow e-mail authors time to change their minds.

"Turn on Undo Send in Gmail Labs under Settings, and you'll see a new 'Undo' link on every sent mail confirmation," explains Google user experience designer Michael Leggett in a blog post. "Click 'Undo,' and we'll grab the message before it's sent and take you right back to compose."

Leggett warns that Undo Send won't recall a message that has already been sent, it simply holds messages for five seconds to allow time for second thoughts.

Google has even dug up some metrics to justify the addition: A company spokesperson pointed to a study showing that 87% of executives reported they have mistakenly sent or received an e-mail or other electronic message.

Google launched another Gmail Labs addition on Wednesday: the ability to preview YouTube videos, Picasa and Flickr links, and Yelp reviews from within Gmail messages.