They actually have some sort of make up that they can put on people to make them look like they've been crying. Good actors/actresses like Halle Berry cry on their own. Most the time they just think of something sad.
I know a girl who wanted to fake like she'd been crying so she could get off easy for not finishing a project. She had some sort of make up that she put under her eyes and it really did look real... I thought it was kind of stupid though.