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My question is simple, when you have a small lump of sugar, it's hard to break it with your bare hands, but when you blow in it, it appears to be more easy. (it's a piece of advice i learned for my gran, and it works !)

Is it due to humidity of the breath? or the spacing between sugar cristals? or something else?

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By breathing on a lump of sugar, you are condensing the water vapor in the air on the sugar itself; water molecules tend to solvate saccarose and thus they reduce the energy of the interaction between saccarose molecules, making the crystal a lesser bound system, more easily broken with bare hands!

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