A strategy for discovering definitions of unknown words is to see which words or roots or prefixes it contains within it. For example, if you didn’t know the definition of say, preheat, you could look at the prefix, pre and because we know that pre means before and you may already know what the word heat is, you can assume that the definition of the word preheat is to heat before. This strategy is limited to your prior knowledge of words before looking at the unknown word. If you don’t know any words that make up the unknown word, using this strategy is not effective. (I don’t know whether this reached 250 words or not, but I do hope that I have helped you.)