Today we did an experiment about air movement, in which we tried to blow out a candle with various items between us and the candle. The original idea for the experiment came from here. This is an excellent experiment for getting your kids to use their powers of observation and deduction.  The experiment uses common household objects. Here’s what you need:

a candle and lighter

assorted household objects of various shapes. I had three broad types: cylinders (cocoa can, wine bottle, rootbeer can); solid and non-round (cereal boxes, storage containers, piggy banks, toys); and those with holes (Connect 4 game with pieces removed, a piece of window screen).

I began by lighting the candle and having both boys blow it out with nothing blocking their way. We talked about how the wind (the blowing) blows the candle out. I then stood the box of cereal in front of the candle, and asked them to predict what would happen when I blew on the cereal box. Would the now sheltered candle go out? I blew and then we talked about why the candle did not go out in this case.

Then we moved on to the Connect 4 game. I first left all the pieces in and blew—and the boys predicted correctly that the candle would not go out. Gareth spontaneously said, “But if we take the pieces out, the air can go through!” so I removed most of the pieces in the middle row and blew again, and the candle went out.

Next we tried one of cylinder-shaped objects—a hot chocolate mix jar. I put it in front of the candle, and asked the boys for predictions. I blew on the jar, and the candle (which was completely sheltered by the jar)blew out! Daegan found it so shocking he laughed out loud.

I then had the boys pick various objects out of the pile to test, and after trying to blow out the candle past them, I sorted the objects into two groups— “Yes, the candle blew out” and “No, candle did not blow out.” Daegan recorded the results on a sheet of paper.

Once we had done a reasonable number, I had the boys come look at the two groups.

I asked them some questions: What do the “yes” objects have in common? Do they have holes in like a screen? How are the “yes” objects different from the “no” objects? Do you notice anything about their shape? Daegan exclaimed, “They’re all round!” We then talked about how air moves around round objects—it hugs the shape of the object. So when you blow on the side of a pop can, the air current ‘splits’ (some hugs each side of the can) but then joins back together at the back side of the can to blow out the hiding candle.  This is called the Coanda effect.

I then extended the experiment by having the boys hide behind various-shaped objects, while I “blew” (waved a heavy book to make wind) at them. Not scientifically legitimate, but they had loads of fun collapsing down (pretending to be blown out) behind the round water bottle. :-)

If you try this experiment at home, a few tips. Try to select thinner cylinders, like wine bottles or drinking glasses—the fatter containers were MUCH harder to blow air around. Bring the candle close to the edge of the table or counter so you can squat down and line up your blow at the level of the flame. And practice before-hand; I found the candle needed to be as close as possible to the blocking object. Lastly, if your kids want to try doing the blowing, have them practice blowing out the candle with nothing in the way first—or better yet, use a hair dryer or similar so that they can aim a strong current of air correctly. I learned too late this was not the best experiment to do with a strong-willed, but asthmatic, 4 year old who wants to try everything himself! :-)