The term 'FILLS UP' can be solved in a crossword puzzle by four different words: REFUELS (7 letters), GASES (5 letters), SATES (5 letters), and GASSES (6 letters). REFUELS means to add fuel to a vehicle, while GASES and GASSES refer to the act of supplying a space with a gas or gases. Finally, SATES means to satisfy someone's hunger or thirst. All of these words are related to the concept of filling something up, whether it be a vehicle, a space, or an individual.