Three cups of water two 4L water

There are now three cups and no ticks, respectively 8l,5l and 3L. How to make 8L and 5L cups with 4L of water without the help of other tools.

will be 8L Sheng 4L is simple, with 5L and 3L manufacturing two 2L on it. After that, the 8L Cup will not move any more. Using 5L and 3L manufacturing 4L is the key is to create a 1L on it, 1=5-4, but this is not feasible, to make is 4,1=3-2=6-5, so the problem is solved, can be in the 3L of the Cup remaining 1L into 5L and then pour 4L can be. This 1L can produce a 1L of water, can also make a 1L of space, here is the need to create 1L water. In solving this problem, the idea is to think of 4=1+3 or 4=5-1 from 4=2+2, and here is the use of the 1+3, the same 5-1 can be used.