You will need: A big sheet of white paper (we used a strip of butchers block paper), a few different food colourings, a few snails and a bit of patience.
The kids dipped the snails from our back yard in the food colouring and put them down on the paper. After the initial shock the snails began to move about the paper and create art for us. The kids had fun putting leaves and grass around the outside of the paper to try and tempt the snails to go one way or the other. The snails seem completely unharmed by the food colouring and we see the "used ones" happily around the place still with coloured shells days later.