I know there are already a few knex catapults here on Instructables and mine might not be that much different from them, but I still wanted to add it an see what you think of it.
I do hope you don't mind if I make any spelling or grammatical mistakes, because I'm from the Netherlands.
Now continue on to the first step for a list of the parts you will need.
Step 1: Pieces Needed
here is a photo with a list of pieces needed to build this device
Step 2: Making the Base
it doesn't need any explaining, just follow the pictures
Step 3: Making the Sides
again just follow the pictures
Step 4: Making the Arm
note that in the second picture the three parts are combined using three white rods
Step 5: Adding It Al Together
slide the yellow rod through the hole in the yellow connectors and add the spacers, note the two grey clips at each side of the yellow rod
Step 6: Adding the Rubberbands
attach the rubberbands on the front of the arm with a white rod. then pull them underneath the arm and attach them at the back of the catapult by putting them around the white rod in the back.
Step 7: Finished
congratulations you finished the catapult. it wasn't that hard, was it? now there is only one thing left to do, telling me what you think of it by leaving a comment. I hope you like it and I hope to make another instructable soon about a marble ballista made from knex.