Compressed air rocket

Bore a hole through the cap of a plastic bottle, push a plastic drinking straw through it and seal the joints with adhesive. This is the launching pad. Make the rocket from a four-inch-long straw, which must slide smoothly over the plastic straw. Stick colored paper triangles for the tail unit at one end of the straw, and at the other end plasticine as the head. Now push the plastic tube into the rocket until its tip sticks lightly into the plasticine. If you press hard on the bottle the projectile will fly a distance of 10 yards or more.

When you press the plastic bottle, the air inside is compressed. When the pressure is great enough, the plastic straw is released from the plug of plasticine, the released air expands again, and shoots off the projectile. The plasticine has the same function as the discharge mechanism in an airgun.