Whisky joke

A man asked his bartender for an 18 year old dram of whiskey. The bartender turns around and pours a dram of an 8yo whisky. 

The man takes a drink and says "what the hell is this?! I asked for 18yo whisky. This is an 8yo whisky" 

Slightly impressed, the bartender turns around and poured a dram of 12yo whisky, thinking maybe he won't notice. Much like the last dram he takes a drink and says "Hey man, this is 12yo whisky. I wasn't born yesterday. I'm going to ask you one more time for a dram of 18yo whisky." Now very impressed he turns around and pours the man what he asked for, apologizes to him and advises him that it is on the house.

A homeless dude sees this entire exchange and says "Hey man! Teik a drink a thiz." And slings him a glass. The man takes a drink and spits is out violently. The homeless guy asks "Do you no what dat iz?" The man screams "THIS IS PISS!!!"

The homeless guy asks "Can you tell me how old I am?"