Why do icecubes crack when they're in a glass?

It’s a hot summer day and you go in the house for a nice glass of (insert favorite beverage). Upon getting your beverage you discover it’s cool, but not cold enough to quench your burning thirst. “No problem,” you say, “I’ll just drop in a couple of icecubes.” You get the icecubes from the freezer, drop them in the glass and snap! pop! Little cracks and fissures appear all over the cube.

Why is this?

My guess is that the water molecules in the icecube are contracting, as that’s what water molecules do when they change from a solid state to a liquid state. The contraction happens in certian areas and similar to plate tectonics there are areas where the molecules go in opposite directions. That’s the place for a crack to appear. Am I close?

I actually saw an icecube shatter doing this. I dropped it into the glass and crack!, I had a dozen or so little fragents.

Sudden immersion in water at a very different temperature means the different parts of the object expand at different rates and internal tensions make it crack. For the same reason you can easily crack or shatter a glass by putting it suddenly in boiling water.

sailor is quite right. “Thermal stresses”, as they are known, can be extremely strong. They can be a really problem (requiring lots of work to get around) in any situations in which uneven temperature changes are present.