One way to build your preschooler's vocabulary is to emphasize the names for things. For example, if you pass a fire engine, talk about its function and how it may have gotten its name.
Be playful. Make up a riddle to encourage interest in and enthusiasm for words. For example, “What do we call that round yummy circle that tastes good with milk? Right! A cookie.” Then have your child make up his own riddle for you to guess.
Words = Fun!