thanks to moongrower and tugbrethil for the info on onion sets

the boiling water makes the cells swell and burst. all the cell contents then spill out and that makes the mush. hope that helps abit.
dont you just love kids and their questions?

Erratic and insufficient water supply while they're growing causes this - although some varieties are more prone to this problem.

Flowery potatoes are far more likely to do this than waxy ones. As Bamboo has already said this is lack of and erratic watering during the growing season. As the over dry potato is cooking it absorbs too much water and turns to mush. Solution is to steam that particular variety of potatoes.

spot the scientist mode rather than the horticulturist :o)
I read that as a tissue/cell problem not a growing one.

Ha ha Seaburngirl - you're still right though, it is a tissue/cell problem - but with a simple cultural cause! Yet more evidence for the nature/nurture debate, lol.