It is known that a family of meromorphic functions is normal if each function in the family shares a 3-element set with its derivative. In this paper we consider value distribution and normality problems with regard to 2-element shared sets. First we construct an example, by use of the Weierstrass doubly periodic functions, to show that a 3-element shared set can not be reduced to a 2-element shared set in general. We obtain a new criterion of normal families and new Picard-type theorems. The proofs make use of some results in complex dynamics. More examples are constructed to show that our assumptions are necessary. Recently, Liu-Pang [14] improved Schwick's result as follows. THEOREM B. Let F be a family of functions meromorphic in D and a, b, c distinct finite numbers. If f −1 ({a, b, c}; D) = (f ) −1 ({a, b, c}; D) for every f ∈ F, then F is normal in D.