Which word is the preposition in: "The cat sat under the mat."

1. Prepositions express spatial or temporal relations, such as in, under and behind. 2. "Under" shows where the cat sat in relation to the table. 3. Cat and table are nouns, and sat is a verb. **Why not the others:** Cat and table are nouns, and sat is a verb. _Source: Wikipedia, "Adposition", section "introduction" (revision 1374713894)_