Directed percolation

Directed percolation is like a game that shows how water can move through a path. It starts with a big square board. In the board there are two types of spaces - open and closed. Open spaces are like holes in the board and the closed spaces are like walls that the water can not move through. In the game, you place small pieces of paper (called directed edges) on the open spaces to show the path that the water can flow through. The water will then follow these edges and can only flow in the directions they are pointing. At the end, you see how far the water can travel and if it reaches the end of the board - it has passed the percolation test.