I am going to show you how to make a bungee cord powered catapult that will fling a skeleton mask out of a hiding place to scare your trick or treaters.

Here's what you're going to need:
-Wood Scraps (I used less than 3 feet of wood that were scraps from other projects)
-PVC Pipe
-Wooden Dowel that fits into the PVC
-Bungee Cord
-Garbage Can
-Halloween Mask (try one that's light weight)
-assorted screws and a drill.

Heres the environment that this was used in to get you familiar with what I was making here. I would have two witches standing over a fogging cauldron (garbage can). Trick or treaters would be brought up to the scene by a guide. The witches would start chanting, and at the right moment, the guide would set the catapult off, sending the skeleton mask flying out of the cauldron towards the trick or treaters...

Below, there is a sequence of pictures of the mask popping out of the cauldron.

It scared several people this past Halloween...

Step 1: Make the Base of the Catapult...

The first step of making a pop up skeleton catapult is to make the base for the catapult. I made this out of scraps of wood. I used two pieces total, each about a foot long. Size really doesn't matter too much, but the wood scraps should be strong enough to hold some pressure.

Here I just took a the smaller piece of wood (1x2") and screwed it into the larger piece of wood. I used two screws on each end of the wood strips. Be sure to drill pilot holes so the wood doesn't split.

Step 2: Add the PVC Pipe

The next step is to attach a PVC pipe to the base. This PVC pipe will hold the dowel that you will be working with later on. Make sure the PVC pipe has a larger diameter than the dowel, and the dowel will be able to slide in and out of the PVC.

I attached this to the base with two screws...

Step 3: Drill a Hole in the Wood Dowel...

For this step, all you are going to do is drill a hole in the wooden dowel near the top. Be sure the hook at the end of your bungee cord will be able to fit through the hole.

Step 4: Cut a Slot in the PVC...

The point of this step is to make a slot to hold the pop up skeleton mask in it's down position until the mechanism was set off.

Basically, I cut a slot near the top of the PVC. The slot should be L shaped going from the top of the pipe about an inch down. I made the cut by drilling several holes, and then wiggling the drill until all of the plastic was cut into the shape I wanted.

There are probably better tools to do this, but with working with what I had, my method worked.

Step 5: Add a Screw Into Dowel...

In this step, I drilled a screw into the dowel that lined up with the L-shaped cut that was made into the PVC in Step 4. This screw would go into the slot, then turn into the bottom of the L. When the dowel was turned while attached to a bungee cord, the mechanism would set off sending the mask flying.

After this step, the core of your pop-up skeleton catapult mechanism is pretty much done. I assembled what I had done at this point, and what comes next is setting up the catapult in the environment you will be using it in. How I did that comes next...

Step 6: Setting It Up (Part 1)...

This catapult was made to make a skeleton mask pop up out of a witch's cauldron. I didn't have any cauldrons, so a garbage can substituted. I had to punch a few holes in the can to get my apparatus working properly, but in my opinion it was worth it.

The first thing I did was to attach the base of the catapult to the bottom of the garbage can. This way, when the bungee cord was taut, the base wouldn't go flying up with the rest of the catapult. I attached the base by doing nothing more than using some wood screws going in through the outside bottom of the garbage can into the base of the catapult. I used six screws total for this.

Step 7: Setting It Up (Part 2)...

Next I drilled a hole in the side of the garbage can about the same height as the screw in the wooden dowel. I threaded a string through the hole and tied it to the screw (with several good knots). After the string was attached, I turned the dowel into the locked position of the PVC's L-shaped slot. I then took the bungee cord and attached on end through the hole in the dowel. I took the other end of the bungee cord and attached it to the handle of the garbage can. I wrapped the bungee cord around a few times until the cord was tight enough to launch something out of the garbage can.

The catapult is now in working condition. At this point you should be able to test it out by pulling the string. It should launch and be pretty easy to reset by pushing the dowel back down and turning.

Step 8: Add the Mask

The final step is to add the mask and put the catapult in whatever scene you want to scare people with. I found that putting a 500mL water bottle over the dowel sent the mask flying farther. I guess there is more surface area touching the mask on the bottom of the bottle compared to the end of the dowel, so it flings the mask farther. I stuck the mask over the bottle, and the catapult was ready to go.

For a final touch, I added a strobe light at the bottom of the garbage can and put another hole for a fog machine to spew fog into the can, making it look like a steaming cauldron. The fog and the strobe hid the mask really well.

In the end, this scared several trick or treaters. Especially the ones that saw me right away in my werewolf mask and kept on repeating "You didn't scare me! I saw you!" As they were saying this, I would lead them up in front of my witches, tell them to ring the doorbell, and when they would look back at me, I would pull the string. It worked very well! And the best part is it cost me basically nothing to make since I had everything in my garage already!