The world's machines derive their designs from one or more of the most basic types of machines, called simple machines. There are six types of simple machines: lever, inclined plane, wedge, screw, pulley, and wheel and axle. With common household items, you can build examples of each type of simple machine to better help children understand how the machines work.
Build a lever consisting of its two parts, the bar and the fulcrum. A see-saw, or teeter-totter, is an example of this simple machine that kids can easily relate to, so make a miniature model using a pencil and a separate eraser. Set the eraser on a flat surface and lay the centre of the pencil on top of it. Encourage the kids to push down each end of the pencil alternately to mimic the motion of a see-saw with two riders.
Use a triangular wooden block and a small toy car or truck to demonstrate an inclined plane. Stand the block on a flat surface so that it resembles a steep hill. Place the toy vehicle at the bottom of the "hill" and push it up to the top point, explaining to the children that it's easier to drive the vehicle up the inclined plane that it would be to lift the vehicle to the high point. Push the vehicle back down the hill and explain that it's easier to drive down an inclined plane than it is to lower the vehicle any other way.
Demonstrate the wedge simple machine with the triangular wooden block and a lump of clay. Use the triangle as a knife to divide the lump into two pieces. Explain to the children that a knife is an example of a wedge, which forces two objects apart.
Show the children a jar with a screw-on lid, without the lid attached, to demonstrate how screws work. Screw on the lid to demonstrate how the screw design of the jar holds the lid in place. Note, also, that the screw is a type of inclined plane.
Use a rope, a bucket and a stuffed animal to explain the concept of a pulley. Hang a rope over the branch of a tree so that the ends of the rope hang over each side of the branch. Tie one end of the rope to the handle of a bucket and place a stuffed animal in the bucket. Pull the other end of the rope to raise the bucket to the tree branch. Explain to the children that the pulley made it easier for you to lift the stuffed animal to the branch.
Stick a pencil through the centre of a CD to demonstrate a wheel and axle. Hold either end of the pencil and roll the CD along a flat surface to demonstrate how the axle (the pencil) enables the wheel (the CD) to move wherever the axle goes. Note to children that cars have four wheels and two axles, which move them and their families from place to place.