Turn on the waterworks
Definition 1 of 1
Pro Tip 1/3Emotional Manipulation
Used to describe someone who cries insincerely or exaggerates emotion to influence others. 
Whenever she wants something, she turns on the waterworks and suddenly everyone's on her side.
Pro Tip 2/3Not Literal
The phrase doesn't mean actual water flowing, but refers to tears flowing like water from a tap. 
He didn't actually break any pipes; he just turned on the waterworks after losing his toy.
Pro Tip 3/3Negative Connotation
The expression often implies that the crying is strategic or fake rather than genuine. 
The jury wasn't moved when the accused turned on the waterworks during the trial.