Rinse the whole watermelon under cool water and scrub the rind gently. Dry it with a clean towel so it won't slip on your board. I always set the board on a damp cloth for extra stability.
Lay the watermelon on its side and slice it into rounds about one inch thick. That thickness is just right for holding the fangs in place. Cut the rounds into wedges or leave them whole, depending on how big you want your monsters.
Set the slices on paper towels and blot the surface lightly. Extra juice can make the decorations slide around. I usually give them two or three minutes while I get the candy ready.
On the top front edge of each slice, cut out a small piece of watermelon. This little gap is where the teeth will sit. Keep it small, because a tiny opening grips the fangs better.
Push the fake teeth gently into the opening you just made. They stayed in place nicely for me, with no glue or toothpicks needed. If a pair feels loose, nudge it in a bit deeper.
Now comes the fun part. Place the candy eyeballs on the red flesh above the mouth, in any pattern you like. I let the kids do this step, and their wonky, uneven placements made the monsters look even funnier.
Move the monsters onto a platter and pop them in the fridge until serving time. Cold watermelon tastes much better and holds its shape well. Bring them out just before the party so the candy looks fresh.
Let guests pick their favorite monster and admire the silly faces. Remind everyone to take the fangs out before biting in. I like to snap a quick photo first, since these look adorable on a party table.