Have some nail polish that seems to be dried up and lost its fluidity?  Do not throw it out!  Do not go out to buy a new one!  You can still save your nail polish with this simple trick:

 Pour about half a cap of acetone/nail polish remover into your nail polish.  Tightly screw on the top and shake well.

After this, your nail polish will coat those nails with the same shine and finish as if it were brand new!