So you fancy yourself a pretty good cliff-jumper. You always personally inspect your landing spot for hazards and the necessary depth. Good. But here's the thing, how do you know how much depth you need unless you know exactly how high you're jumping from? With NASA smarts, that's how.

Our old friend, former NASA JPL engineer Mark Rober is back with another video, and this time he's using some simple math to help you accurately gauge the height of the cliff you're standing on before you decide to jump off it. It goes like this:

- Toss a rock straight out from the cliff and use a stopwatch to time how long it takes to hit the water.
- Multiply that number by itself, and then multiply that by 16.
- That should be almost exactly the height of the cliff in feet. If you want that in meters just multiply by 0.3 to give you the rough answer.

How does it work? It's just a simplified version of Newton's second equation. It's d = 16t^2. *Slightly* less accurate, but infinitely easier to do in your head as you're staring down at your possible death. Mark actually made it even more painless by creating a simple, free iPhone app called Cliff Height Timer. All you have to do is hit start and stop and it'll take care of all the math. Pretty sweet.

Mark also notes that this same trick can be used to estimate the height of a tree or structure, by throwing a rock as high as its peak, starting the timer at its apex, then stopping when it hits the ground. Or you can use it to call bullshit on your friends (or YouTubers) when they over-exaggerate the height of their jumps.

Hopefully this simple equation will prevent some broken ankles and legs. Remember, according to our expert, "You need at least eight feet of water, adding an extra two feet of depth for ever 10-foot gain in cliff height." Respect the math. [Mark Rober]