Your students likely know that the world is mostly covered by water. They might not know how much, and some may not even be able to name all four oceans or all seven seas, but they know it takes a lot of water to cover most of a planet. Even a former planet, stripped of its title because of its small size, would need a lot of water to submerge most of its surface.
What your students probably don’t know about Earth’s water is just how little of it is actually usable by the billions of organisms on the planet. This quick and easy Demo Science science demo will help them wrap their tiny little noodles around the growing scarcity of fresh water.
Comparing Apples & Oranges… Er, Water
For this demonstration, all you’ll need is one apple (the bigger the better) and a knife with which to cut said apple. Start by holding up the whole apple and informing your students that it represent all the water in the world, both fresh and salt.
Then, cut the apple into equal quarters. They absolutely must be perfect fourths—in fact, all cuts going forward have to be perfect or the whole thing goes right down the toilet—so you may want to call on someone who does high precision aerospace machining to ensure that your Golden Delicious ends up in exact quarters.
Cut one of those fourths in half (into eighths, if you will), and remind your mathematically-challenged students (*cough* Kevin *cough cough*) that each of those smaller pieces represents roughly 12.5% of the whole.
Cut that smaller piece in half (representing 6%), and then cut one of those smaller pieces into halves again. One of the resulting, tiny pieces represents about 3% of the whole. Inform your students that this tiny hunk o’ apple represents all the fresh water in the world—the rest of the apple represents the oceans.
From there, cut one of your 3% pieces into a one-third and a two-third section. These pieces are approximately 1% and 2% of the whole apple. (That doesn’t exactly shake out mathematically, but if you don’t mention that, your students probably will likely be none the wiser.)
Take the smallest of small apple bits, your 1% analog, and tell the kids that this tiny little piece of the whole represents all of the usable fresh water on Earth. Water that has to be shared by every single living organism on the planet. Discuss what this means, and give kids ideas for how to conserve water in their daily lives.
Whether you want to tell them that they’re inheriting a horrendously damaged planet from a generation of adults who won’t be around the experience the consequences is up to you.