There are six different types of simple machines; pulley, lever, wedge, inclined plane, screw, and the wheel and axle combination. Each of these machines can be used in a variety of ways. A hammer is an example of a lever, a bicycle uses the wheel and axle combination, and a wheelchair ramp is an example of an inclined plane. Combining these simple machines creates a more complex mouse trap.
Lay the first ruler horizontally on the left side of the plywood base. Place the square block under one end of the ruler to create an inclined plane. Glue the block to the ruler to hold it in place. Lay a thread spool on its side. Glue the center of the second ruler to this thread spool to create a lever. Repeat for the third ruler. There are now two levers. Tape the blade from the utility knife to the underside of one of the levers. Ensure the cutting edge of the blade faces down and is able to cut through string.
Set the box on its side so the hinged lid can be pulled up to allow the mouse into the box. Turn a thread spool on its side and place on top of the box on the opposite end of the hinged door. Place one of the small wooden blocks to either side of the spool on top of the box. Glue the small wooden blocks in place. Insert the wooden dowel through the center of the thread spool on the top of the box. Set the thread spool between the wood blocks, with each side of the dowel resting on a wooden block. Tape the dowel securely in place. Verify the wooden spool can turn freely. Punch a hole in the hinged door of the box and tie one end of the string through the hole.
Place the lever without the utility knife attached next to the inclined plane so that the side next to the plane is in the up position. Place the second lever so the end without the utility knife attached overlaps the down end of the first lever. When the first lever is activated, the second lever should move so that the knife will go down to cut a string. Set the box on the edge of the inclined plane so it will tip over and land on the first lever when the mouse crawls inside.
Thread the string attached to the hinged door over the thread spool on top of the box. Pull the thread tightly enough to hold the hinged door so it is horizontal to the box. Continue pulling on the thread as the other end is placed under the lever with the utility knife attached. Tie the end to the spool of thread under this lever. The string should be tight enough to keep the hinged door up. Place the bait inside the box to attract a mouse. When the mouse enters the box, it will fall backwards onto the first lever. The first lever will activate the second lever, which will cut downward and cut the string holding the box door open. The door will close and trap the mouse.