Wash the rocks and leave them to dry completely. Paint takes forever to adhere to a damp stone, so this step matters more than it sounds.

Apply the first coat of white acrylic paint over the entire rock, using smooth strokes in one direction. It will look pink and patchy. That is fine. Leave to dry for 20 minutes.

Apply a second coat in the opposite direction. Still uneven in places. Third coat, same direction as the first. By the third coat the surface should be a solid, chalky white. If any grey stone still shows through, add a fourth coat after another 20 minutes.

Once the white is fully dry, use the black paint or marker to draw a simple ghost face: two oval eyes and a slightly open oval mouth, all in the upper half of the rock's widest face. Keep the features small relative to the rock; the ghost shape is created by the white rock itself, not by an outline.

An adult finishes each rock with a light coat of acrylic sealer spray or a brushed layer of Mod Podge to protect the paint. Let it cure for at least 30 minutes before handling.