paramedicpete's right: place the bottles at an angle in the freezer. It really works.

I was thinking about Pete's post this morning and why it works but it's a complete mystery to me. What the heck is going on when it's slanted that is different from a level container? Egads, did I miss the MythBuster's episode on this???

Slanted allows more room for expansion as the water turns to ice.

(Darn it, wildman beat me to it. Anyway, my long-winded version follows.)

From what I've observed:

When water freezes, it starts by forming a crust at the top with liquid water underneath. I assume this is because ice crystals are less dense, so they float to the top.

In a container, this ice crust forms an effective pressure seal once it reaches a certain thickness. When the rest of the water slowly freezes (and expands) it exerts equal force against the "ice crust" and the rest of the container wall.

For a container oriented vertically, the surface area of the ice crust will be smaller and thicker for a given volume of water. So, the container wall gets the maximum pressure possible -- and may burst.

For a container oriented at an angle (horizontally) the surface area of the ice is the largest possible, and so is thinner and weaker. It is likely more pliable for a longer period, so it may move and bow before it becomes an absolute pressure seal. So, there is less pressure on the container wall, and less chance of failure.

(I warned you, long-winded. Feel free to "flame" my ice theory. It's possible that I am hopelessly snowbound on this.)

I don't believe that it is necessarily tied to ice crystal density as it is temperature conduction. The surface loses its heat quicker, hence ice. The mass (volume) as a whole takes longer to have the cold penetrate (heat loss) at the center.

Anyone?

(Caution! Nerdy and increasingly OT discussion in progress!)

I'll buy that. Air cools more quickly than water. That would certainly apply to an open container, but would likely apply to a closed container as well, as long as there was an air space at the top.

I can attest to the "pressure seal" effect. When my polyethylene rain barrels start to freeze over in a serious way (2" of ice) I break the ice with a spade or axe and the water squirts upward quite impressively. I suppose some sort of foam "tube" (like the ones you wrap water pipes with) would keep a hard seal from forming. Gotta try that.

I've seen a picture where it was at just the right temperature & pressure that it froze into a spike. It happens occasionally with ice trays but this was the first time I'd seen it happen on a large scale.

Water hates compression. That is why depth charges work so well.