With just 4 marshmallows, 7 barbecue skewers, a rubber band, a plastic teaspoon and a strip of masking tape you can have yourself a maths, science and English lesson.
Have a little chat about what a catapult is, where they've seen it and introduce your materials. Get the children to have a little turn trying to assemble it themselves then show them one, which may or may not look like the one in my illustration.
As an English lesson, students can write a procedure on how to construct a catapult. In maths, measure distances of appropriate objects thrown from the catapult. In science, talk about potential to kinetic energy.
This is probably best taught to children from Year 3s upward.