Building a dam for a school project is generally done to illustrate why dams are built and how power is generated as a result. Building one is simple, inexpensive and fun.
Prepare an area for the experiment--over a sink, in a tub, or on a water table. Gather all your materials nearby.
Cut the top off the milk carton and lay the carton on its side. Measuring up from the bottom, go 1/2 inch up and poke a hole with the nail in the center of the side (hold the carton from the inside to keep it from collapsing). Moving up from that hole go up another 1/2 inch (1 inch total) and poke another hole above the first. Move up another inch (2 inches total) and poke a third hole in a straight line above the first two. Move up 2 more inches (4 inches total) and poke a final hole aligned with the rest.
Cover the four holes with a single strip of masking tape.
Mark the inside of the carton with a water line near the top to use when doing the experiment more than once and always fill to that line.
Fill the carton with water to the line and set the carton on so that the water will run into the drain.
Remove the tape quickly to allow water to drain from all four holes at the same time. Make note of how far each stream travels in the sink. Also, note what happens as the water runs out and the water level drops.
Re-tape the carton and do the experiment again. This time, though, only uncover one hole at a time, measure the distance the water travels and re-tape it. Move onto the next hole and do the same, refilling the carton to the fill line each time.
Provide answers to why the water from the hole closest to the bottom travelled farthest. That hole has the greatest mass of water on top, which equals water pressure, causing it to travel further and faster. This is how hydroelectric plants work in conjunction with dams. The water pressure at the base of the dam turns huge turbines that generate electricity.
When poking the holes, try to keep them all the same diameter.