Use the mouse to select your sand color and drop it to make patterns. Space bar also works for both selection and dropping. Drag the color picker to where you want. C key hides or shows the color picker. Up arrow makes the grains bigger, down arrow makes them smaller. Works best in full screen mode because the color picker is pretty small.
Paddle2See for the original game :) As the title says, all I did was add a size changer with up/down arrows.