Each is served a pint and by coincidence/a blatant disregard for food safety rules, each pint has a fly in it.

The Englishman shoves the pint back to the bartender, demands a refund and leaves in a huff.

The Irishman picks the fly out of the drink and enjoys his pint.

The Scotsman grabs the fly and starts yelling at it “spit my beer out you little bastard, spit it out!”