The design for this tongue depressor catapult from a really fun book called Mini Weapons of Mass Destruction by John Austin. I know it sounds dangerous, but with a little parental supervision most designs are quite safe for kids. And so much fun! It's engineering disguised as mayhem!
The basic construction is simple. You stack seven tongue depressors together and use two rubber bands on either end to hold them together. Then take two depressors and attach them with a rubber band on one end. Spread the two depressors into a "V" shape, and insert the stack lengthwise to make a "T" shape. Use another rubber band to secure the depressors together into your catapult. (The book gives clear, step-by-step instructions with diagrams.) For the original design, the author suggests using a hot glue gun to attach a bottle cap as a launcher, but I find it easier to just attach a disposable spoon with another rubber band or two. No electricity needed!
You can make the catapult with popsicle sticks too, though I find a stack of just five is enough. The popsicle sticks are thicker, and a bit harder to work with, but you can get the job done. I also like to make the coat hanger slingshot from the book too, and challenge kids to see which works better. The book has a whole chapter devoted to slingshots, catapults and trebuchets, so kids can have a lot of fun comparing results.
Then it's time to load a mini-marshmallow and get shooting! I like to give kids dixie cups that they can use to make towers. It's fun to knock them down. Dominos are also fun! For some Angry Birds action, paint your marshmallows red and green (or use leftover Christmas-themed ones, like I did here).
Another great catapult design that is easy to make and fun to tinker with comes from a great YouTube video I found years ago. This shoebox catapult is always a hit.