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.