Have you ever clicked "Send" on an email and regretted it a fraction of a second later? Whether you were angry and typed something you shouldn't have, clicked "Reply All" when you wanted "Reply," or just accidentally clicked the "Send" button before the email was finished, your first instinct is to find and click an "Oops!" button. Unfortunately, there is no such button for email ... or is there?

While I can't tell you how to unsend email you might have sent already, I can give you four options to set up your email so you can undo them in the future. Taking a few minutes now can save you a lot of embarrassment later.

Gmail 'Undo Send'

Google's Gmail is an incredibly popular email system thanks to its powerful and customizable inbox. It also has more features on the back end than you can shake a stick at, and it's adding more all the time.

One feature Gmail has had for years is "Undo Send," but it was experimental and buried away. Now, it's front and center, and you just need to enable it.

In your Gmail account, click the gear icon in the upper-right corner and choose "Settings." On the "General" tab, scroll down and find "Undo Send." Click the checkbox for "Enable Undo Send" and choose the cancellation period.

You can choose a cancellation period of 5, 10, 20 or 30 seconds. This is how long you have to realize that you didn't mean to send that email and call it back before it gets delivered. Once you have that set, scroll down to the bottom of the screen and click the "Save Changes" button.

Now when you click the "Send" button on a message, you'll see a message appear at the top of your Google inbox with an "Undo" link. As long as the message is on the screen, you can click the "Undo" link and the email will reappear ready for editing or deleting.

Of course, maybe you want a grace period longer than 30 seconds or you use a different email service. I've got something for you, too.

Browser plug-ins

There are a few browser plug-ins that offer unsending features. The first one we're going to look at is Criptext. Right now, it's just for Chrome and Safari, but Firefox and Outlook program versions are in development.

When you enable Criptext on an email, it encrypts the message and any attachments, tracks it so you know when it's been opened, and gives you the option to "recall" it. You can also set messages to expire after a certain amount of time.

UnSend.it is a service that works with more browsers and email services than Criptext. It does everything Criptext does, except encryption. You will also need to configure your email service to run through UnSend.it's servers.

Once that's done, you can send email and attachments to anyone. If you decide to unsend the email, just click a button in your email service and they won't see it anymore. You can also track when someone opens the message or set it to self destruct after an amount of time.

Both of these services work by turning the text of your message into an image, then sending that image on to your recipient. Because the message is served from their servers, they can delete the image and it will disappear from your recipient's inbox.

The email will still be there, but it will be blank. Unless your recipient took a screenshot, they won't have the information. If you really want to take things to another level, however, keep reading.

Secure messaging

Most free messaging systems aren't really that secure, which is why a former NSA employee decided to take matters into his own hands. The result is called Virtru.

Virtru is a plug-in for Firefox and Chrome that works with Gmail, Yahoo and Outlook.com. There's also a plug-in for Outlook 2010 and 2013, along with apps for Apple and Android.

The free version of Virtru lets you send secure messages to anyone. If the other person has Virtru, they can open the message in their regular email. Otherwise they'll need to get Virtru's free Web viewer.

If you upgrade to Virtru Pro for $2 a month, it adds message revoking and expiring, along with the ability to see and control message forwarding. Learn more about Virtru's features and step-by-step instructions on how to use it.

Email isn't the only place you can unsend things you regret. Find out how you can take back embarrassing posts on Facebook.

NEW YORK — Ride-hailing company Uber just completed a round of funding that values them at a whopping $51 billion — or more than four times the value of rental car companies Hertz and Avis combined.

The latest funding came from Microsoft and the investment arm of Indian media conglomerate Bennett Coleman & Co., according to the Wall Street Journal, citing anonymous sources. Uber raised close to $1 billion in the latest round, bringing its total funding to more than $5 billion, the WSJ reported.

Uber has 3.5 billion shares outstanding, according to documents filed with Delaware's Secretary of State, where it is incorporated.

"We filed to authorize this new funding more than two months ago" Uber said in an e-mailed statement. "We aren’t commenting on additional speculation."

Hertz is valued at $7.8 billion. Avis Budget Group, which is also publicly traded, is valued at $4.6 billion.

Uber carries the loftiest start-up valuation in an already frothy tech economy, beating out room rental company Airbnb, which is valued at $25 billion, and Snapchat at roughly $15 billion.

Investors have been lured by the company's fast-paced growth. Uber, which was founded in San Francisco in 2009, is doing business in 58 countries across Europe, Asia and the Middle East. The company recently said it plans to invest $1 billion to expand in India.

Uber is not without risks, however, including challenges from workers over their status as independent contractors.

The company has long kept its expenses down by claiming that it's merely a technology company matching drivers with riders, not a car service company with vehicles to maintain. The saddles drivers with bulk of expenses such as maintenance, insurance, gas and the vehicles themselves.

Some employees have sued to gain employee status, a designation that could substantially increase the company's costs.

Uber has also faced opposition in cities like Paris, because the service matches passengers with drivers who don't have the proper licenses. In New York, some local politicians have expressed concern the impact of Uber's growth on already congested and polluted streets.

Last week a proposed cap on Uber and other for-hire vehicles in New York was put on ice after the start-up agreed to "not flood the streets" until questions about the impact of its growth on traffic can be determined.

A flying DeLorean?! No, just another Terrafugia car-plane concept

While the likes of Google focus on driverless cars, Terrafugia continues to look much further into the future with flying cars.

The company has been talking about its heli-car hybrid the TF-X since 2013 and has just released a video showing its new exterior design in action.


Now for the first time, Terrafugia has announced that a one-tenth scale model will be tested at the MIT Wright Brothers wind tunnel.

"The wind tunnel test model will be used to measure drag, lift and thrust forces while simulating hovering flight, transitioning to forward flight and full forward flight," Terrafugia said.

The video shows the TF-X leaving a garage and taking off from a landing pad. It's equipped with twin electric motor pods, two wings and two helicopter-style propellers.

Four passengers can be carried a maximum of 500 miles, while the TF-X is capable of flying up to 200mph.

Unfortunately, Terrafugia predicts that development of the heli-car hybrid will take 8-12 years.