Popular science projects for elementary and middle school students include the magic glass of water project and making a simple electric motor. These project ideas, along with many others, are described in detail on Education.com.
The magic glass of water project teaches elementary and middle school students that an "empty" glass of water is actually full of air. The project is very simple; it involves filling one quarter of a drinking glass with water, placing a piece of cardboard over the rim of the glass, then inverting the glass while pressing the cardboard firmly against the glass rim. After letting go of the cardboard, it stays attached to the glass, apparently defying gravity. This is due to the surface tension of the water and the pressure exerted by the air inside the glass.
Another popular project for elementary and middle school students is the creation of a simple electric motor. Following the instructions given on Education.com, students need to create a coil of wire that has two lengths of wire pointing out of opposite sides of the loop. Students can then carefully balance this coil of wire on two needles that are attached to a battery and place a magnet on top of the battery, directly underneath the coil of wire. If the system is set up correctly, then the electric current flowing through the circuit causes the wire to keep rotating when it is set in motion.