A clown called Bubo works on his computer during the 4th Latin American Clown Congress in Guatemala City, July 25, 2012. REUTERS/Jorge Dan Lopez You clicked send. Oh crap.

When you send a no-take-backs email — maybe an admission to a secret crush, or accidental reply-all — there's an instant pang of regret. It feels like there's no going back.

Meet Gmail's Undo Send feature, a lifesaving little hack buried in the Gmail Labs settings. It gives you a 30-second window to "undo" sending an outgoing email.

You just have to enable it first.

Here's how it works:

1. Click the gear icon in the top-right corner of your Gmail window and select Settings from the dropdown menu.

2. Select Labs from the row of tabs.

3. Scroll all the way to the bottom where you see Undo Send and click Enable.

4. Hit Save Changes at the bottom.

5. Breathe easy.

Now when you send an email, the yellow dialogue that displays "Your message has been sent" will also give you the option to Undo. Click it, and the email will reopen, un-sent, in the composition window.

It defaults so that you have 10 seconds to click before the Undo button disappears, but you can adjust that window of opportunity. Go to Settings > General > Undo Send, and select a cancellation period up to 30 seconds.

We weren't the first to discover this cool trick. Jimmy Tomczak, founder of beach accessories company TOMBOLO, recently tweeted me a link to an article by former Mashable editor and current Sprout Social editorial director, Samuel Axon. Gmail's interface has changed since Mashable's tutorial was published in 2010, so we updated the steps.

Note, features in Gmail Labs are experimental and may change, break, or disappear at any time. You've been warned.