Prepositions are a small set compared to other parts of speech. While prepositions are a closed set, nouns are ever increasing as science and technology advance and new words are needed to describe new concepts. Other parts of speech are being added to as well (for example, to be “ENRONed”), though not as rapidly. Table 1 shows the word-count-by-part-of-speech for words in Roget’s Thesaurus.