Footprints on the Sand

What you need

• Bottle full of dry sand
• Clear balloon
• Funnel
• Water

Instructions

1. Inflate the balloon and stretch it over the neck of the bottle. Invert the bottle so the sand pours into the balloon. Pinch the balloon shut then gradually let the air out of it.
2. Pour water through the funnel into the balloon until all the sand is wet and there’s a little water in the neck. You’ll need to squeeze and tap the balloon and stretch its neck to get the water in.

Results and Explanation

When you squeeze the balloon the level of water in the neck goes down instead of up. The volume of the wet sand expands when you squeeze it and the water flows in to fill the gaps between sand grains.

Squeezing the sand causes each layer of grains to shift and the layer below to move the opposite way. These tiny movements create gaps in the bulk of the sand which the liquid can fill.

These experiments have not been specifically safety tested for home use but we believe them to be safe if the instructions are followed. Adult supervision or direction is recommended as appropriate. All experiments are carried out at your own risk.