Fill a shallow pan with about half an inch of the mixture.
On a rainy day, hold the pan outside for a few seconds. (Or you could let water drip from your hand into the pan, but going out in the rain is more fun).
Let it sit, undisturbed for a few hours (or until the next day).
Use a fork to carefully remove the wet spots. Set them on a plate to dry.
After they're dry, examine their shape and size. Why do they look like balls and not like a "raindrop" shape?
You may want to try this on a variety of days - a day when it's barely raining, a day when you have a steady shower, and if you dare, a day when you have a downpour. What similarities and differences do you observe between the raindrops collected on each of these days?
Older students could measure each of the drops they collected and then analyze the data; graph it, determine the mean, median, and/or mode.