This is a very simple trick pumpkin that gets great results. It's cheap, easy, and very effective. The build is easy enough that even kids can do it and the trick is so reliable that even small children can scare the pants off adults. It works so well because everyone has seen one of these plastic pumpkins before and they're primed to reach right at the opening -- usually with candy!
I made one last year and took it around with my kids trick-or-treating. I'm an adult so many people answering the door looked at me suspiciously when I held out my "pumpkin". I just pretended to be collecting candy for my "other kid, sick at home". Even with suspicion raised, the trick worked great and I got invited into several parties. We 'test drove' the pumpkin at the annual Pumpkin Festival in Hartsburg, MO. Here's some of the reactions we got in broad daylight.