In artificial intelligence and cognitive science, the structure mapping engine is an implementation in software of an algorithm for analogical matching based on the psychological theory of Dedre Gentner. The basis of Gentner's structure-mapping idea is that an analogy is a mapping of knowledge from one domain (the base) into another (the target). The structure-mapping engine, or SME, is a computer simulation of the analogy and similarity comparisons.
As of 1990, more than 40 projects had used it [Falkenhainer, 2005]. R.M. French said that structure mapping theory is "unquestionably the most influential work to date of the modeling of analogy-making" 
The theory is useful because it ignores surface features and finds matches between potentially very different things if they have the same representational structure. For example, SME could determine that a pen is like a sponge because both are involved in dispensing liquid, even though they do this very differently.