After they find their seats, the waiter comes in to collect their order.
One of the friends asks for a jar of lime juice while the other asks for five pints of beer. After some time has passed, the waiter appears with a jar of lime juice and two pints of beer.
Why did this happen?
The waiter interprets their gestures, intended to be Roman numerals, as Arabic numerals.
Holding up one finger: Roman numeral 'I' becomes Arabic numeral '1'. As a coincidence, the waiter gets this right since both mean 'one'.
Holding up two fingers makes a 'V' which is the Roman numeral for 'five'. The waiter gets this wrong, so to speak, because he understands it as 'two'.