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For those of us familiar with a certain magical boarding school,
the name basilisk is most likely associated with giant, deadly yellow eyes,
and maybe a blood stained sword.
But the real basilisk isn't a snake; it's a lizard, and it can't kill you by looking at you.
A real life basilisk does have one special power:
it can run on water.
But it's not magic that gives it this ability, it's science!
And because it's science, you might wonder why a lizard can do it, but a human can't.
Well, it turns out that you can run on water, with a little effort,
or rather a lot of effort,
and a spaceship.
You can walk on solid ground like dirt, wood, or concrete because the attractive forces between the molecules are strong enough to hold all of them in place.
In a liquid, these forces are weaker, so instead of supporting your weight, the molecules just slide around one another and out from underneath your feet.
So, if you want to walk on water, you're going to need a way to use physics to find a way around that problem.
For really tiny animals, all they need to do is take advantage of water's strong surface tension,
an attractive force between molecules on a liquid.
Arthropods, like water striders and certain species of spider also have tiny hairs that repel water on their many long legs.
That lets them literally stand still on top of the water, with none of their body piercing the surface.
But that only works for animals that weigh less than a gram or so.
Above that, their weight is just to much for surface tension to be able to support them.
They might be able to float, but they can't stand or walk on water.
Some of these animals can run on water though.
They just have to use another technique to keep most of their body mass above the surface.
They have to generate a force that acts against the gravity pulling them down.
There is a pretty simple way of doing this.
You slap the water with your foot really fast,
then do a kind of swimming stroke below the surface with your leg.
Repeat with the alternate leg until you get to the other side of the body of water,
or you sink,
or you give up and just start swimming.
The challenging part is pushing against the water hard enough to keep the animal's body above the surface,
while also minimizing the drag caused by pulling the animals foot out of the water.
Each animal that can do this has its own tricks of the trade.
Basilisks, for example, have a sort of fringed toes that give them a boost in surface area during the slapping phase,
and generate an air pocket during the stroke to keep drag low.
For a medium to large sized common basilisk, most of the support comes from the stroke as opposed to the slap,
but the largest animals to run on water get much more out of the slap:
up to 55% of the force they need to keep themselves almost entirely above the water.
The record-holding bird, the western and Clark's grebes, are about 10 times heavier than the runner-up,
and can cover up to 20 meters during what's known as rushing,
a mating ritual where they run across the water really quickly.
Grebes have specialized feet too, with lobes on the side of each toe instead of webbing, and flat toe lobes.
And when they draw their feet out of water, they fold them up into a sideways motion.
But as you might have noticed, humans aren't two kilogram birds,
so we need to generate way more force than that to run on water.
Researchers have actually done the math and it turns out that the average person would need to slap the water at an impossible 30 meters per second,
and they'd have to do it four times every second.
That would require an average power output 15 times the maximum of what typical human leg muscles can do.
To cut that velocity down to something that's actually reasonable, say 10 meters per second,
you need the bottom of your feet to have a combined area of 1 square meter,
and it would be pretty hard to run fast, or even run at all with shoes that big.
So how on earth can a human run on water?
By leaving Earth.
In a study published in PLOS ONE in 2012,
because yes, there's been a study on this,
researchers strapped people into a harness over a wading people to simulate lower gravity.
Wearing small, rigid fins, all the subjects were able to stay on top of the water for at least 7 seconds at 10% of Earth's gravity,
and the highest gravity where any one subject could still run on water was 22% of Earth's.
That means, assuming you're kind of fit, you could theoretically run on water on the moon, which has about 17% of Earth's gravity.
Actually, you could run on water on any solar system body that isn't the planet or the sun.
Of course, we haven't found any bodies of water that you can run on anywhere else in the solar system,
so we might never get to use this particular talent.
But, maybe it's enough to know that technically, we could.
Thanks for watching this episode of SciShow. If you're interested in answers to more weird questions you might never have thought to ask,
you can go to youtube.com/scishow and subscribe.
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