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Today, I took two crumpets out of the freezer, which had become stuck to each other where moisture between them had frozen. They were two separate crumpets, the top one stuck fast to the bottom one, so they didn't go into the freezer attached to each other in any way.

And it made me wonder, what causes the crumpets to become stuck? Why don't you just have a crumpet, a thin sheet of ice, and then the other crumpet? What's going on at the chemical level that explains why these two crumpets became stuck to one another?

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The crumpets you have frozen together are very porous and convoluted at a microscopic level. One would then imagine that this means that any water vapour condensing on them as they start to freeze will percolate somewhat into the surface of the material and then freeze. The interior will stay warmer longer than the surface so this will help percolation. (Imagine an ink drop spreading into paper and you get the idea of percolation). On freezing water crystals become connected and thus many of these stick the two parts together. Rather like a sort of very fine velcro. However, most all foodstuff already contains a lot of water surrounding lipids, proteins and carbohydrates. Thus hydrogen bonding of condensed water to these intrinsic waters may also be important at the molecular level.

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