The 4,000 year old fart joke

The oldest joke discovered (so far) was written on a Sumerian tablet in 1900BCE, and it's about bottom burps. You know - guffs, grunts, air biscuits, poots, etc. Farts! The joke goes - "Things that never happened since time immemorial - a wife did not fart in her husband's lap". Not sure we get it, but it's to do with farts so.... still funny. 4000 years on!