Sandglass Puzzle Game

How to play this game?
I'd like to boil the eggs. I need to put them in boiling water and take them out after 3 minutes.
But I don't have a watch, only have a five-minutes sandglass and a two-minutes sandglass.
How can I get three minutes with 5 minutes-sandglass and 2 minutes-sandglass?
5 - 2 = 3 , Aha! I think I can get it from this.
Let the sand of both sandglasses flow at the same time.
It's been two minutes. The sand of the 2 minutes-sandglass dropped all down and three minutes of sand remained on top of 5 minutes-sandglass.
It takes three minutes for the sand remaining on top of the 5 minutes-sandglass to flow down. Now put the eggs in boiling water.
The remaining sand at the top of the 5 minutes-sandglass is flowing down.
It's been three minutes since put eggs in boling water. Now let's take out the boiled eggs.