I wrote this project originally for Craft: http://blog.craftzine.com/archive/2011/10/project_gory_brain_cap.html
The project has also appeared in Make: http://makeprojects.com/Project/Gory-Brain-Cap/1465/1
Fitted cap, without adjuster
Craft knife or seam ripper
Acrylic paint, red and black
Bowl, approximately same size as cap
2 tubes of latex or acrylic caulking (not silicone)*, ivory or bone color
Step 1: Remove the cap's brim by carefully ripping the seams with a craft knife or seam ripper.
Step 2: Place the cap on newspaper. Paint the cap with dark red paint (a mixture of red paint and a little bit of black paint). Let dry.
Step 3: On newspaper, create a work stand for your cap by placing a bowl upside down over a tall jar. Place a plastic bag over the bowl, and the cap over the plastic bag.
Step 4: With a craft knife, cut the tip of the caulking tube to have an opening approximately 1/2" in diameter. Load the tube into a caulking gun.
Step 5: Caulk convolutions on one half of the cap to create one hemisphere of the brain. Repeat on the second half of the cap.
Step 6: Let the caulking dry for approximately one to two hours until the surface loses tack. Wearing gloves, lightly press on the convolutions to slightly flatten the surface.
Step 7: Let the caulking cure for 24 hours.
Step 8: Once the caulking has cured, mix water with red paint and a little bit of black paint, to a thin, runny consistency. Use a brush to dab paint mixture onto the caulking, focusing on crevices. Apply a second coat if needed. Let dry.