Let’s do a quick rundown of things all kids know about trees: they’re tall, there’s leaves at the top and roots at the bottom, they “drink” water out of the ground, they’re good for climbing and building houses in. That should pretty much cover it. If any of your students doesn’t know all of those facts, send them back down a grade and tell ‘em to come back when they’re good and learnt up.
Now, to the point of this particular Demo Science science demo: reconciling two of the above tree facts. The leaves—way, way up at the top o’ the tree—need the water that the tree drinks out of the ground to grow and continue their photosynthetic processes. But the roots that drink up that water are way, way at the bottom o’ the tree. How does the water get from the bottom to the top?
The Short Answer: Very Slowly
The mechanism behind trees’ water transfer is called “capillary action.” To demonstrate this process, you’ll need two drinking glasses, something to serve as an elevated platform for one of the glasses, water with which that same elevated glass, and some paper towels. If time is not on your side, you can perform an abridged version of this experiment by exchanging the paper towels for plastic tubing. (See “The Fast Version” below.)
Fill one of your glasses to the brim with water and carefully place it onto its platform, right at the edge of said platform. (But not so close to the edge that it could fall off if bumped.) Place the empty glass on the table (or whatever is the base for your experiment) and slide it right up next to the platform so that it’s very close to, if not actually touching, the full glass.
Tear off two paper towels, twist them together into a rope kind of thing, and bend the rope-ish into a U-shape. Stick one end of your paper towel rope into the full glass, and the other end in the empty glass. Have your students observe what happens over the course of the day. This is a good place to note that this demonstration will take a while, and you may want to start it off first thing in the morning so the kids can check in on its progress multiple times throughout the day.
The Fast Version
If you don’t have all stinkin’ day to wait on a marginally-interesting science experiment, you can speed it up thusly:
Everything up to the paper towels should be the same. Then, instead of ropeifying paper towels, use a short length of plastic tube. Place one end into the full glass, then draw on the other end to start the water flowing through it. If you’ve ever siphoned anything in your life, that’s basically what you’re doing here. Put the free end of the tube into your other glass and observe as the water transfers much, much more quickly.
Note: Though faster, this version of the experiment is far less interesting, as it’s over pretty quickly. To jazz it up a little bit, I recommend putting your full glass one side of your classroom, the empty glass on the other, and getting a really long length of tube (or multiple shorter tubes joined by liquid-tight I-Flex connectors) and running it over/under/around/through any obstacles so that the water will wend its way across the room. You’ll have to put your full glass on a much higher platform this way—and it will require far more suction to start the siphoning process—but will be much more visually interesting, which is your best bet for getting today’s ADD-addled kids to pay attention to literally anything for more than three seconds at a time.
I Was Told There Would Be Science Involved…?
Whichever way you do this experiment, it’s a miniaturized and simplified version of the capillary action that a plant’s roots use to move water from the underground roots to the skyscraping leaves. All further references refer to the OG-slow-G version. (If you’re doing the fast version, just imagine everything sped up a bit.)
Before long, the end of the paper towels in the water will have soaked up enough liquid that it will start to seep down their length and into the waterless glass. In just a few minutes, water will start to accumulate in the bottom of the no-longer empty glass. It travels very, very slowly, oozing through the countless spaces between the fibers of the paper towel. Plant roots do the same basic thing, transferring water molecules between from one fiber to the next until the water has “climbed” all the way up the tree to the thirsty leaves. In your face (very slowly), gravity!