For those wondering how a spark plug would work: https://en.wikipedia.org/wiki/Ninja_rocks

Large parts of Minneapolis beg to differ.

You just need to be slightly smarter than the average opportunistic thief. Hit the glass very hard in the corner where it is held most firmly so that you get a large force distributed over a small area, that is the largest stress. The same thing goes for smashing a car window. I have a small spring loaded device on my keyring for exactly this purpose [1].

A slightly smarter thief might carry a hammer specifically designed to break double glazing [2]. Mind you carrying that in the UK could count as 'going equipped' if you don't have a plausible excuse.

Otherwise just use a heavy pointed rock in the corner of the window.

But of course most burglars don't break windows, they take advantage of open windows, doors than can be forced, sliding doors that can simply be lifted out of the track, and other flaws in building security.

[1] Like this: https://www.smartasaker.no/no/artiklar/resqme-belteskjarer-a... [2] For instance: https://www.amazon.co.uk/dp/B00EFZC62Q