Surf’s Up Wave Machine https://physicsfunshop.com/search?keywords=surfs up Bart rides a surfboard between two immiscible liquids which are sorted under gravity by density with the densest (dyed water) on the bottom and the lightest (mineral oil) on top. Bart’s board has a density between that of the two liquids so it stays at the boundary layer, and a capsule of air above Bart’s head keeps him upright. The motorized rocking mechanism in the base is powered by one C cell battery and produces the