What do houseflies eat?

My little sister just turned 5 and she got a bunch of bug catching gear for her birthday. She woke me up this morning to show me that she caught a housefly! I told her after she looks at it for a while, she should let it go so it doesn’t die in her little “bug box” but she wants to keep it. I told her if she keeps it she has to feed it and such so we’re trying to figure out what houseflies eat.