I can see that Peter is trying his best. We are still working in the potato corner of the walled garden. 'Plants produce scent for attraction as well as defence. The lemon-smelling plants all share the essential oil limonene.'
But they're not lemons, I say.
'Why do you find that surprising?' he asks, incredulously. 'Why is it surprising to call something with lemon in it, lemon? If you use the word red for describing the property of a plant, that's not surprising. We are simply describing the red property that disparate plants share from a human point of view. The colour red,' he says, very slowly and emphatically, 'Is... just... a... gene. As is a particular scent.'
I don't think that this line of enquiry is really holding Peter's attention, and he busies himself with the lifting of potatoes. We speak of related things.
Scents which are repellant to insects may be attractive to humans, for example. The essential oil geraniol is good in soaps, 'But when you eat it, it's absolutely vile!' Lemon sorbet is very good as an amuse bouche but lemon verbena sorbet is even better.
A couple of weeks later and I have prepared the killer question.
'What makes a lemon a lemon?'
'The same thing that makes a tomato a tomato.'
Next time in Science with Peter
Where does all the manure go?