Your five-year-old knows it: When it comes to building sand castles at the beach, wet sand is much better than dry. If you fill your bucket with dry sand and turn it over, you’re likely to end up with a tumbled ruin, not a proud tower or turret. Any junior civil engineer will tell you to use damp sand from the water’s edge to construct proper parapets. We'll learn why wet sand sticks on today's Moment Of Science. Fortunately for beach-goers everywhere, a group of physicists at Notre Dame tackled this sticky problem. They discovered that wet sand sticks together for exactly the same reason water beads up on wax paper: a phenomenon called surface tension.