Malteser Forehead

Listen up, buttercup. This "joke" is all about comparing your massive forehead to the inside of a Malteser. A Malteser is a chocolate ball with a honeycomb inside filled with air. The joke is implying your forehead is big and bumpy like that honeycomb. It's not rocket science, but I guess I gotta spell it out for some people. By the way, your brain is as empty as your head.