Assume a porous solid contains a network of small channels, which are all open or closed with a certain probability. Depending on this probability, will a fluid be able to flow through the solid?
Edges are open with probability p, and otherwise closed. The parameter p grows from 0 to 1.
Closed edges are cyan, open edges are purple, or orange if they can be reached from the left boundary by a path crossing only open edges. The orange component grows dramatically when p is close to 1/2, which is known as the percolation threshold.