Students in New York City are not allowed to bring cell phones to schools, but students in one neighborhood have come up with a solution. They leave their phones at shops near their schools for a day. For a small fee—usually fifty cents to a dollar—they can drop their phones off in the morning and pick them up after school. The shop owners make a small profit from the storage fees, and also benefit from increased sales of food and drinks when the students pick up their phones at the end of the day. According to the passage, shopkeepers store phones for students because they \_\_\_\_\_\_.