Re: Why are the grains of sand so uniform in size?

The sand you see on a beach was deposited there by waves.  Waves deposit
material when they wash on the beach and also pull material away when the
water washes back out.  There are all sizes of rock particles.  The size 
of the material that gets deposited and removed depends on how powerful 
the waves are.  The water washing on to the beach has more energy than the
water washing back out.  The stuff that's small enough to be washed on to 
the beach by the incoming waves but too big to be carried away by the 
returning water is what stays.  In many cases this is the size of sand.

On some beaches you can actually see the difference that more powerful
waves make.  During the summer there are generally less powerful storms, 
and the beaches are sandy.  During the winter, the more powerful waves
bring larger material on to the beach, and it looks more like gravel has
been put down.