You have a large number of friends coming over and they all get thirsty. your first friend asks for 1/2 a cup of water. your second friend asks for 1/4 a cup of water. your third friend asks for 1/8 a cup of water, etc.

how many cups of water do you need to serve your friends?

First things first, convert all the fractions to ones with the same denominator. 2,4, and 8 can all be divided (or go into) 8 evenly. Make 8 the common denominator. To get the new numerator, multiply the same number as you did to get the new denominator.
1/2-->4/8 (4 multiplied by 2, which is the original denominator, gave you 8 as the new denominator. Now, multiply 4 times 1, which is the original numerator to get the new numerator of 4.)
1/4-->2/8 (2 multiplied by 4, which is the original denominator, gave you 8 as the new denominator. Now, multiply 2 times 1, which is the original numerator to get the new numerator of 2.)
1/8-->1/8 (Since 8 is already the denominator, no changes need to be made.)
Now that you have the common denominators, add the numerators. When you have common denominators, you are allowed to add or subtract the numerators as is. 
4/8 + 2/8 + 1/8 = 7/8. (4+2+1=7)
You need 7/8 cups of water to serve your friends.

The answer is not exactly known but it is less than 1 cup of water.