Depends on the test. E.g. some IQ tests measure the size of your vocabulary. IIRC, the reason why this works is that people with a higher IQ tend be to quicker at learning the meaning of a word from its context, and therefore accumulate a larger vocabulary. That makes the size of your vocabulary adequate as a rough proxy for IQ - but only within your age group, since people older than you have had more time to accumulate a large vocabulary.