It turns out that you can, with some effort, make a hole in ‘puncture proof’ tyres. I do not recommend this, of course, and it does take a surprising amount of work and incompetence, but I have managed it as follows:
Pump tyres up to 4 bar, (one bar under the maximum, but the person who worked this out probably thought the tyres would be used as they were meant to be: on flat roads. Under a lighter bike).
Take Xtracycle to garden for family picnic.
Before going home, load Xtracycle with remains of picnic, a bag of compost, several gardening tools, a large plant pot and some work boots.
Decide Youngest Son is too tired to ride home on his own.
Put youngest son on already overloaded Xtracycle. Strap his bike on to tow it home as well.
Consider leaving some things but then realise that means coming back to get them in half an hour. Decide you can’t be bothered.
On the way home, drive up a very steep, badly surfaced road, putting entire weight of (overloaded) Xtracycle, Youngest Son, and and a rider that is ‘cough’ kilogrammes heavier than strictly necessary, onto the back wheel.
Mash pedals to get up hill.
Fail to notice one particularly sharp bit of gravel embed itself into the thickest part of the back tyre under the pressure.
Continue riding (and probably overloading) the bike for the next two days.
Result: one unscheduled visit to the bike shop for a replacement tyre and inner tube.
I guess that’s why the tyres are sold as ‘puncture proof’ rather than ‘idiot proof’.