These are ripple marks in beach sand, made by the wind. Similar ripples can be seen in deserts, and even in large playground sandboxes. What causes the ripples to be so evenly spaced?

Stirring Sand

I ran across the idea for this week's experiment (which is really more of a challenge), while researching another question. The more I thought about it, the more I liked it. It involves the unusual behavior of sand in a glass of water.