Get an ice cube tray and a toothbrush. Put the toothbrush into an empty square, and fill it up with water.
Carefully place the tray in the freezer. You may need to balance the toothbrush on something to keep the brush in the right position.
Your toothbrush should take a few hours to freeze. Once it has frozen, carefully take it out of the ice cube tray without cracking the ice.
Now put in the toothbrush holder. You don't want the ice to melt, so do this just shortly before your victim goes to brush their teeth.
Watch your victim get brainfreeze!