This is a K'Nex Trebuchet I built one day. It really doesn't take that long, and it is pretty simple to operate. It weighs about one pound, and can fire a good three feet befort it touches the ground. It also doesn't need wheels, like most catapults or trebuchets. I use marbles for my ammunition, but you can use whatever is lightweight and fits in the container.

Step 1: Materials

You are going to need:
6 red, 5 inch pieces
5 orange, 5 inch pieces
12 yellow, half circle connectors
10 yellow, 3 and a quarter inch pieces
8 purple, 2 and a half inch pieces
2 white, 1 and a quarter inch pieces
6 orange, straight connectors
9 white, full circle connectors
4 gray, 7 and a half inch pieces
2 blue/purple, full circle/vertical connectors
2 gray stoppers, with a hole one end and a connector on the other
3 C cell batteries
Duct Tape
These are all the materials you will need to make a K'Nex Trebuchet.

Step 2: Base

This step handles one of the most important parts of any trebuchet, the base.

1. We start by taking 4 white connectors.
2. Use 4 orange pieces to connect them and make a square.
3. Take 4 yellow pieces and one white connector and make a plus, then insert this into the middle, making a square with an X in the middle.
4. Take 4 yellow connectors and slide them onto opposite orange pieces.
5. Now, connect 2 purple pieces together using an orange connector.
6. Take the connected purple pieces and attack them to a yellow connector, so that they are one below the vertical space and are horizontal. Do the same on the other side, but with an orange piece. This step is confusing, so be sure to look at the picture.

Step 3: Platform for the Trebuchet Arm

This step is where you build a platform for your trebuchet's swinging arm.

1. Use 4 gray pieces and attach them to your yellow pieces, so that they are vertical.
2. Take 4 yellow connectors and attach them upside down, so that the flat part is facing upward and the curved part is facing downwards.
3. Use 2 red pieces to connect opposite sides of the platform. It is important that you connect them as shown, so that the trebuchet's arm can swing.
4. Attach 2 white connectors to opposite red pieces.
5. Attach the white connectors together with a yellow connector.

Step 4: Stabilizers

After a few tests of my trebuchet, I discovered that it was very unstable. I decided to add some stabilizers to the front to keep it from tipping over.

1. Take 2 red pieces and attach them to different white connectors on the base, so that they are pointed forwards.
2. Attach your 2 blue/purple connectors to the ends of the red pieces.
3. Attach a red piece and a yellow piece together using an orange connector. Now, connct one end to the blue/purple connectors so it runs to a yellow connector on the top. Attach it there as well. Do this again so that both blue/purple connectors are connected to a yellow connector.

Step 5: Trebuchet's Arm

This is the most important step in your K'Nex Trebuchet.

1. Attach a yellow piece to one end of a white connector, and an orange piece to the other.
2. Attach a second yellow piece to the first yellow piece using an orange connector. This is called the arm.
3. Take a white connector and attach 4 purple pieces onto it, in the same as the base. Attach them to four different corners. I call this part the container.
4. Attach the container onto the end of the arm that you made.
5. Slide the arm onto the yellow piece that connects the two white connectors on the platform. Make sure that the container is pointed towards the stabilizers.
6. Center it, then attach the 2 gray stoppers onto either end, so that it can't move from side to side.
7. Duct tape the three C cell batteries onto the end of your arm that doesn't have your container. This will provide the weight that will make your trebuchet fire.

Step 6: The Swing Controller

This controls how far your trebuchet's arm can swing. Without this, the trebuchet fires much less effectively.

1. Connect a yellow piece to a purple piece using an orange connector. Do this again, so that you have two.
2. Attach these to the white connectors that are part of the platform, so that they are vertical.
3. Connect 2 yellow connectors to the tops of these, so that the flat side is facing upwards and the curved side is facing downwards.
4. Use a red piece to attach these two yellow connectors together, in the same way as the base.
5. Attach 2 white pieces to the yellow connectors so that they are pointed away from your stabilizers.
6. Attach 2 more yellow connectors to the ends of these.
7. Pull back the trebuchet's arm, then connect the two yellow connectors together in the same way as the red piece.

Step 7: The End

This is all it takes for your very own K'Nex Trebuchet. I haven't tried it with any more weight on it, so I don't know how it will hold up. Feel free to modify this design in any way you see fit, and if it works, please comment so that others can enjoy your improvements.