Penny Science

I learned about this from a child's magic book. It fascinated me quite a bit as a child, yet the science behind it can be quite an educational moment. You will need a few cups of different sizes and a large stack of pennies. Put the cups somewhere that is OK to get wet. (I did mine in the sink) Then, fill the first cup with water. It must be full to the point of spilling if one more drop was added. Then, rub your finger around the rim of the cup to make sure that the rim is dry. After that, slowly place pennies in the water (carefully so as not to make waves.) You will see the water level bubble up out of the cup eventually. (It should not take too long if you filled the cup enough.) Let the kids discuss why they think the water is bubbling over the top of the cup, but not spilling. If you want, you can use this chance to teach them about the cohesive (bonding) properties of water. (It bonds to other water molecules so it stays together instead of spilling - to a certain point.) When the water eventually spills over, you can count the number of pennies that you put in the cup before it spilled. Then, repeat with a different size cup. After doing this a few times compare the number of pennies you got into each cup. Did larger cups hold more pennies?