You should ask before you drink someone else's beer.
Brent is at his acquaintance's house and sees that there is a beer in the fridge.
Brent wants to drink the beer in the fridge.
Brent asks his acquaintance if he can drink the beer in their fridge, and they say yes, so he does.
Brent grabs the beer from the fridge without asking and drinks it all.
Brent's acquaintance is happy Brent asked them if he could have the beer instead of just helping himself.
Brent's acquaintance thinks Brent has no manners if he will just take things from their fridge without asking.