Many years ago at LHR.
Q : "Dad ... what?'s the difference between a Trident Two and a Trident Three?"
A: "The number of engines...."
And at CPT :
"Did you know the London - Cape Town flight is faster than the Cape Town - London, must be because it's downhill."
"Oh I think it's because the English petrol is better than the stuff they put in it here."
Still my best though :
"Do I have to get to the airport before my 'plane goes?"