Volume!

Volume is defined at the amout of space that an object can hold. Most people dread having to find the volume of a shape, although if you think about it, its just the area multiplied by the height!  if you had the same shape stacked on top of each other you would be able to find the volume!
 it would be the area of the base (the sides that are congruent in shape and parallel) times the hight.  For example, volume of a rectangular prism is V=lwh or V=Bh where B=lw.  For a triangular prism its the same as a rectangular prism just divided by 2 cause a triangular prism is half rectangular prism.

  If having to find the volume of this pool its not very hard area of base (decagon) multiplied by the height (or how full you wanted the pool)  You would want to make sure that you don't over fill it or under fill it!  You would want to keep track of the water going in and be prepared for the water bill that summer!