Water filter - easy question?

Question

i made a water filter for a science project i used sand, gravel, pebbles, cotton. can you please explain to me why these WERE good choices?

Answer 1

A filter is a device that separates components of a mixture by virtue of size difference. For a water filter, you need substances that would allow water to flow through while blocking solid impurities.  

The substances you used were all inert to water, i.e. they do not react or get dissolved in water.  If you arranged them in layers (TOP: gravel, pebble, sand, cotton:BOTTOM), so much the better since you are removing the bigger particles at the top layer, then proceed with smaller particles as you reach the bottom layer.