Mixing is a stronger statement than ergodicity. Mixing asks for this ergodic property to hold between any two sets formula_35, and not just between some set formula_14 and formula_2. That is, given any two sets formula_38, a system is said to be (topologically) mixing if there is an integer formula_39 such that, for all formula_40 and formula_41, one has that formula_42. Here, formula_43 denotes set intersection and formula_44 is the empty set. Other notions of mixing include strong and weak mixing, which describe the notion that the mixed substances intermingle everywhere, in equal proportion. This can be non-trivial, as practical experience of trying to mix sticky, gooey substances shows.