Appropriate grades or age groups:
3rd and up

Estimated time to complete this project:
2 days

Materials:
White glue, sand, bathroom scale.

What is the hypothesis of this science project? What does the experiment aim to prove?
Discovering what effect grain size has on the strength of bricks.

Instructions:
Sandbox sand: sift some with a kitchen sifter so that you sort it into large grains and fine grains. Make a brick mold about an inch long by 2 wide; this can be a form made from legos on top of some wax paper, or rubber hand soap or candle molds, anything, so long as all the bricks are the same size. Larger bricks take longer to cure, add a day per inch. Use a teaspoon to measure out equal amounts of Elmer's or similar white glue for each brick, mix the glue well with the sand samples and make identical size/shape bricks. They will air-cure overnight, 24 hours at most, in a dry place. Make one brick from only the finest grains, one of only the coarse grains, and one mixed half and half coarse and fine.

When dry, take them to the bathroom scale, lay each one in turn across a pencil or wooden dowel rod or similar object on the scale, then slowly stand on the brick until it snaps, note the weight it took to do this. The strength of the bricks depends on the void spaces between the grains, and how much glue "mortar" can get into those spaces to lock the grains together. Won a blue ribbon. Surprising strength in these bricks: the strongest took over 100 pounds to break, the differences between types are dramatic. Real builders use similar methods to grade and produce stronger concrete structures and road surfaces.

Other comments or suggestions:
Other sources for sand in different grades are craft stores and aquarium stores/pet shops.