[laurieg]

The "making a sandwich" demo is definitely a classic.

Here's another I like:

Give groups of kids 10 identical looking items and a balance to compare the weights. Pre-teach how a balance works to compare the weight of items. Ask them to put the items in order from lightest to heaviest. You're basically asking them to come up with a sorting algorithm. Usually, after much experimentation, someone will come up with an algorithm that works. You can work together to try to write down the steps of the algorithm. You can also explore more mathematical ideas like transitivity when comparing different sorted sets of items.