Some examples of working models for science projects include a pinhole camera, a hovercraft and a baking soda volcano. The hovercraft and volcano projects are simple enough for upper elementary and middle-school students. The camera is best for high-school students.
Construct a pinhole camera from an oatmeal container with a lid. A hole poked through the bottom of the box serves as the camera's aperture, which lets light hit photographic paper attached to the container's lid. The paper serves as film. Cover the hole with a makeshift shutter until it is time to take the photo. Then, remove the cover to expose the film. After removing the film from the camera, develop it in a pitch-black room, using photographic chemicals.
A hovercraft model consists of a plywood disk and a vented plastic disk bolted together with plastic sheeting in between them. The plastic forms a skirt that inflates when air is blown into the disk. Place the disk on a smooth, slippery floor. An air compressor or leaf blower produces the air needed to lift the hovercraft.
Build a volcano by molding clay, dough or paper mache around an empty soda bottle. Fill the bottle partway with water and a few drops of dish detergent. Add baking soda to the water. Demonstrate the eruption, which is caused by carbon dioxide, by pouring vinegar into the water and baking soda solution.