What's the highest mountain?

Answer? It depends.

Oh, okay, the answer on earth is Mount Everest, standing an amazing five-and-a-half miles high. Pretty impressive, unless you live on Mars, where there is a mountain called Olympic Mons that stands 15 miles high.

Why is Olympic Mons so much bigger than Everest? Another way to ask this question is: Why aren't there any mountains on Earth fifteen miles high like Olympic Mons? How about thirty miles high? Or a thousand?

The answer is: gravity. Gravity sets an upper limit on how high a mountain can be. Here's why.

Imagine you are the base of a mountain and you are holding a brick on your head. The top of the brick is the top of the mountain. Let's add another brick or two. Maybe three or four more. How are you feeling now? Let's add a hundred bricks. How about now?

The taller a mountain is, the greater the pressure it exerts on its base. That's because, thanks to gravity, all that weight is constantly pressing down on the lowest parts. After about five-and-a-half miles of mountain, the base is so squished it starts to turn into a fluid. That means you can't go any higher.

On Mars, however, gravity is roughly 2.7 times weaker than on earth. That's the same factor by which Olympic Mons is higher than Everest, about 2.7 times. Less gravity equals less pressure on the base. So if you really want high mountains, go to the planet with the weakest gravity. The view will be terrific.