So here are a few fun little projects for you (the kids are totally optional):
Oobleck (a classic)
Set the stage by letting your subject put their hand in a bowl of water. Note how the water reacts as a fluid. How motion is permitted no matter the speed. Now have the subject put their hand on a table. This is a solid (obviously) and note how no matter how fast or slow or how hard or soft you push, the table provides equal and opposite force keeping your hand still. It seems silly to go over these obvious actions, but this understanding is crucial to showing why Oobleck is so unusual (it's a non-newtonian fluid).
Make Oobleck with this recipe (if you are working with a kid, let them measure out everything and put it together with your instruction).
I suggest using some green or red food coloring. Once you have the oobleck, remember what you previously determined about solids and liquids, and then explore the behavior of your new alien creation.
Send Letters With Disappearing Ink
Ruby suggests two possible scary set-ups for this project: leaving secret wolf paw prints, or writing letters to Halloween ghosts.You can order disappearing ink online, or make it using these instructions or these other instructions. Make sure to discuss the chemistry behind what is happening, and how it might impact other things in the world around you. Ask them to come up with ideas of when they might want or need to use something like disappearing ink.
Dry Ice Fog and Fun
Make sure to use proper safety precautions when handling dry ice, and communicate these precautions to any kids involved. Dry ice and hot water can make a cool, creepy fog. Try using these instructions for a basic dry ice fog.
If you want to step up your dry ice game, Steve Spangler has some great ideas for cool experiments with dry ice. The following video is an example of some of his ideas, no copyright infringement is intended.
Yes, all of this is fun to play with, but it is also really interesting science. Talk to the engineer-in-training about different forms of matter. A great example to discuss is water. The solid form is ice, the liquid form is water, and the gas form is steam. When H2O changes from an ice cube, it melts to water before it becomes steam. Dry ice experiments are a great example of sublimation, where matter is able to change directly from solid to gas forms. Here is some more scientific information about dry ice if you want to know more.
Electronic Special Effects
If you are looking for an electronics project, this website has some instructions on how to make a rustling leave special effect to spook some of your trick-or-treaters. Be sure to use proper precautions when using soldering irons, or just use a breadboard instead. If you don't want to go through the whole process of making your own special effect, get one of those tacky (read: awesome) animatronic Halloween decorations and take it apart. Figure out what the different parts do, and why they work. Does it use a light sensor? What does the sensor look like? How does the animatronic make sound? How does it move?
Whatever projects or experiments you decide to try, encourage kids to ask "why", and "how" all of these experiments work. If you don't know the answer (or even if you do) go to a computer or a library or a text book, and help them learn how to look up the answer. Perhaps the most helpful part of experiments is teaching them how to question the world around them. It's that questioning attitude that is the foundation of a great engineer or scientist.
PS. What are your favorite ideas for creepy science experiments and projects?