As you can imagine, she's not getting very far. First she tries posing the ancient Chinese question: What is the sound of one hand clapping? "Piece of cake", says Bart, holding one yellow hand in the air and smacking his three fingers onto his palm (it works, have a go......).
Lisa tries again: "If a tree falls in the forest and no one is there to hear it - does it make a noise?"Bingo! Bart's eyes become wide, unfocused saucers of wonder as he attempts to figure it out and his brain starts to feel all psychedelic and bendy with the strain. Well, I have been remembering Bart in that moment because recently I had one similar.
I was out for my morning walk when I encountered a large party of teenage kids out on a school field trip. We were crossing the road from opposite sides and as they streamed past me, I noticed that every single one of them - male and female - was wearing a pair of jeans. There were about 40 of them.
"That's interesting." I mused casually. In high-fashion terms, jeans are supposed to be over, but they really have become the default with the young now, just as they did with my generation in the '70's.
Then I remembered I was also wearing jeans myself that day, and that really got me thinking, because if every Western teenager and even old birds like me have them, then how many pairs of jeans are there in the world?
And then my pesky brain pursued the idea, like a foolish dog following a ball onto the road and into the path of an oncoming car, pondering, "So how many pairs of all kinds of jeans are there? "And that's when the weirdness hit. Aaaaah! Stop the thoughts! My head feel funny! But still the thoughts continued.....
Every member of my immediate blood family, from my three-year-old great-niece up to my octogenarian mum, owns at least one pair of jeans, and most have several. I quite dread to think how many pairs my trendy older nieces own each.
So apart from the odd nun, at a very conservative estimate, just about every citizen in Australia, the UK and America must have one pair of jeans. Priests wear them. Doctors wear them, on duty. Add in Japan, Russia, Europe and China's burgeoning youth culture and the numbers start to spin out into the kind you need to be Stephen Hawking to understand.
As the concept of the world jeans reserve hit me that morning, it could send you mad, I realised, so I forced myself to stop.
I wondered instead, for the rest of my walk, about that falling tree and that empty forest......
Rwanda, Nov 2006 - Even in Africa the blue jean saga continues.