The joke:
What did Harry Potter say at the gas station?
Expensive Petroleum
The joke plays on a pun from the Harry Potter series.
"Expensive Petroleum" sounds just like "Expecto Patronum," the spell Harry uses to summon a protective Patronus charm.
Simple Breakdown:
Spell: Expecto Patronum (fights off dark forces).
Pun: Expensive Petroleum (fights your wallet at the pump).