Spread trash bag on table to protect work surface.
Place tray on top of protected surface.
Squirt small blobs of tempera paint onto paper plate, keeping the colors separate.
Spray a thick rectangle of shaving cream onto tray. Smooth the top surface of it with the edge of the cardboard.
Use your brush to paint on the surface of the shaving cream with tempera. Do not press the brush into the shaving cream.
Paint stripes, dots or other pattern on the shaving cream, avoiding making pictures or letters.
Stir the painted pattern with a toothpick for 30 seconds, making swirls and figure-eights as you stir.
Lay your paper on the painted surface and pat with your fingers to make sure the paper takes the paint suspended in the shaving cream.
Peel the paper off of the surface and lay it aside to dry for 20 minutes.
Use cardboard to scrape the shaving cream off the paper (over the trash can) to reveal marbled design.