For example, if there is an event of John walking around from 1pm to 2pm, and another event of his walking around from 2pm to 3pm, then there is, by necessity, a third event which is the sum of the other two, which is also an event of walking around. This doesn't hold for expressions like "built a house." If John built a house from time 1 to time 2, and then he built another house from time 2 to time 3, then the sum of these two events (from time 1 to time 3) is "not" an event that can be described by "built a house." Cumulativity can also be used in the characterization of mass nouns, and in the characterization of the contrast between prepositions like "to" and "towards," i.e. "towards" has cumulative reference to (sets of) paths, while "to" does not.