Attention, beach-going children: science has something to say to you. You know that towering castle of bucket-ramparts and seashell turrets you built last week with your dad? Can't touch this. In a very poorly copy-edited but technically interesting paper
, materials scientists from Iran, France, and the Netherlands delve into the physics behind why a little bit of water transforms sand into good castle-building material. They calculate the relationship between the width of a sand-tower's base and the height it can reach and verify it by building sand skyscrapers, which you can see to the right. They estimate that a tower with a base radius of 20 centimeters can get to 2.5 meters high before buckling. The sand sticks together because water builds tiny bridges between the grains through capillary action, but the pressure exerted by the surrounding medium, in this case the air, matters too, the team notes. That ...
