What are the factors that cause a problem solver to become blocked? And what are the factors that allow a person to become unblocked? These are the motivating questions for a set of studies we conducted of individual and joint problem solving. By constructing an isomorph of the classic "water jar" problems (Luchins, 1942) as a dynamic graphic microworld, we were able to identify several factors involved in producing blocked states. By comparing the behavior of individuals tackling the "missionaries and cannibals" problem to pairs of people solving this problem, we have been able to identify ways in which problem solvers operating in a social context are able to overcome problem solving blocks that are difficult for individuals. These studies point to the importance of "reflection" (evaluation of problem solving results) for flexible problem solving. These results may also account for the difficulty in showing learning in "discovery learning" uses of computers, such as the use of Logo, since such uses also often do not encourage students to reflect on the outcome of their problem solving.