When I teach camp, the last experiment on Chemistry day is to ‘walk on water’. I present the kids with a milky-white tub full of water and a secret ingredient. I stick my hand in the tub and pull it out (slowly) so they can see it’s clearly a liquid (as it dribbles off my fingers), and the kids always gasp in surprise when I then smack it with my hand, because now it looks rock-solid.


Their challenge? To step up and across without sinking. Of course, sinking can be fun, too!


NOTE: You’ll see a tub full of water at the end of the line – this helps wash off their feet after they’re done stepping across.


Here’s what you need to do:
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Fill a large tub or kiddie pool with as much cornstarch as you can afford for your group of kids. (You’ll need at least 50-100 pounds to make this worthwhile.) Start to add water, a little bit at a time, stirring with your hands and arms. The faster you stir, the harder it is to stir. Go s l o w l y . You want approximately a 2:1 ratio of cornstarch:water.


When your mixture is nearly ready, grab a chunk of it with your hand – it should form a hard ball that you can’t squish. When you relax your grip, the ball should melt and drip between your fingers as if liquid. Adjust the amounts of cornstarch and/or water to get it just right.



What’s going on? The water-cornstarch mixture is made up of long molecular chains (polymers) that get all tangled up when you scrunch them together (and the slime feels solid). The polymers are so slick that as soon as you release the tension, they slide free (and the slime drips between your fingers). This of spaghetti noodles with butter – they get tangled up, but are still allowed to slide freely.  This cornstarch-water substance is both a solid and a liquid.  Scientists call this a non-Newtonian fluid.


Ever wonder why ketchup doesn’t flow easily out of the bottle? Now you know it’s because the ketchup acts just like the cornstarch-water experiment above. More examples of non-Newtonian fluids are ketchup, blood, paint, and shampoo.


For a smaller scale version of this experiment, try making a small amount and placing it in an empty water bottle. Notice how it acts like a liquid when you gently roll the bottle, but turns into solid bits when you shake it hard.


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Comments

6 Responses to “Walk On Water”

  1. so cooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooolllllllllllllllllllllllll

  2. Carolyn Halvorsen says:

    Thats amazing stuff

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  4. Jia Zhi Chien says:

    it was aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaasome

  5. Danielle Elizalde says:

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  6. I have to tell you – we filled up a kiddie pool with this stuff, and it was a hit of the party! Kids had so much fun sinking into this stuff and feeling it with their feet. Only I’d suggest doing it where you don’t care about getting cornstarch everywhere – like on dirt or grass. I’ll send you pictures!