A bank is getting robbed and one of the robbers tells one of the tellers to give him all of the money. The teller tells him she doesn't have access to it.
Suddenly the phone rings. The robber tells the teller to answer it and not give them away. She picks up the phone and it happens to be her mother. She tells her mother "Is this an emergency mom? Call me when I get home, I could use some help painting." Then she hangs up.
The robbers continue to try to get into the vault but twenty minutes later the police show up with the teller's mom and arrest them all.
How did the police know about the robbery?
(First heard in my 10th grade math class)