When your under legal age for purchasing controlled substances (alcohol, tobacco, etc) you go on a Bum Run. Basically its when city teens buy their alcohol from the homeless. Typically it requires a tip after you get the drink... most homeless are happy to help out!
"I got my fake taken away" "Okay, Well have to go on a bum run"
"Where did you get the alcohol?" "We had to go on a bum run"
"When we went on a bum run earlier the bum went crazy because we didn't give him enough money"
"Earlier today two bums were competing for our 'business' and they got in a fight during our bum run"