There is a ‘grandad joke’ doing the rounds in our family at the moment, which goes like this:

A policeman walks up to a man on the street and says, “I’m arresting you for being a Russian spy”.

The man replies, “I’m not a Russian spy – I’m a shepherd!”

The policeman says, “All right then, I’m arresting you for being a shepherd spy.”

In order for this joke to work, you have to be familiar with the ‘shepherd’s pie”, a traditional favourite for using up the leftovers after a Sunday lamb roast. It can be made with mince, but that tends towards the sloppy, as I seem to remember from my pre-vegetarian days.

Also, the joke teller has to run the words together so that the ‘word boundaries’ are ambiguous and the listener can hear either spy or pie. This is necessary because in English the ‘p’ is actually pronounced differently when it is after an ‘s’ (as in spy) from when it begins a word (as in pie). At the start of a word the ‘p’ has more air, technically ‘aspiration’. It is easy to test if you put your hand up in front of your mouth and say both words. You can feel the puff of air with pie, but it is not there with spy.

We do not normally notice the difference in English – they both just sound like ‘p’, so are one ‘phoneme’. In other languages such as Thai they are two different phonemes which can both occur in the same place, to make words with different meanings.

Possibly the only time the aspiration of ‘p’ actually matters in English is when the difference between shepherd spy and shepherd’s pie is the point of a joke. Go, Grandad!