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Above all, your car should be light. The smaller the mass of your car, the better — every gram or milligram you can shave off of your car's frame is a little further your mousetrap will be able to push your car. Try not to have any extra frame material beyond what's necessary to keep your mousetrap and wheel axles in place. If you see wasted space on your frame, try removing it, or, if this is impossible, make holes in it with a drill to bring its weight down. You'll also want to use the lightest material possible for your frame. Here are just a few suitable ones:  Balsa wood Hard plastic sheets Thin, light metal sheets (aluminum/tin roofing material, etc.) Building toys (K'NEX, Legos, etc.) Ideally, you want your car to be shaped aerodynamically — that is so that it presents the smallest possible surface area in the direction it will travel. Like an arrow, a longboat, an airplane, or a spear, a vehicle that's designed with maximum efficiency in mind will almost always have a long, skinny shape to minimize drag from air resistance. For the purposes of your mousetrap car, this will mean making your frame both narrow (though it will be difficult to get your frame any narrower than the mousetrap itself) and vertically skinny. Remember, to minimize drag, you're trying to give your car the narrowest, smallest profile possible. Try getting down on the ground and looking at your car from the front to spot pieces of the frame that make your car's profile unnecessarily large. Wherever you can, try to use glue in your car's design, rather than nails, pins, or other heavier solutions. For instance, you should only need to use just a few small spots of glue to fix your mousetrap to the frame. Generally, the glue will hold just as well as nails, which can add unnecessary weight. Use super glue not school glue it won't hold as good. Another advantage of glue is that it shouldn't usually affect your car's air resistance. On the other hand, if either end of a nail sticks out of your frame, it can have a minor effect. The only limiting factor when it comes to how light and skinny you can make your mousetrap car's frame is its fragility — if it's too light, it may become so fragile that the action of springing the mousetrap breaks the car apart. The delicate balance between achieving the maximum distance and making your car unstable can be quite tricky to get right, but don't be afraid to experiment. The mousetrap itself is unlikely to ever break, so as long as you have extra frame material, you'll have the freedom to make mistakes. If you're using an extra-fragile material like balsa wood and you're having a hard time getting your frame to hold together, consider adding a small strip of a sturdier material like metal or plastic on the underside of the frame. Doing this increases the structural strength of the car while minimizing the change to its air resistance and weight.

Summary:
Build the lightest frame possible. Make the frame long and narrow. Use glue instead of nails wherever possible. Keep your frame's structural integrity in mind.