of substantial amounts of water, a percolation test is needed beforehand to determine whether the intended structure is likely to succeed or fail.In two dimensional square lattice percolation is defined as follows. A site is "occupied" withprobability p or "empty" (in which case its edges are removed) with probability 1 - p; thecorresponding problem is called site percolation, see Fig. 2.

Percolation typically exhibits universality.