When water freezes inside various tubs and containers, sometimes the ice has large holes in it. They are roughly flying-saucer shaped and filled mainly withy air, but can also contain some liquid water. Dissolved air forms small almost cylindrical bubbles, but how are the large holes formed?

Dear all,

After having cooked about 40 crêpes (I'm French), I put them in a plate (whose edges are higher than the center of it) in order to form a pile.

The crepes at the top of the pile are curved, just like on this picture.

I'm sure it is due to the shape of the plate combined with gravity on top of the pile (the crepes are pulled down in their center of gravity, like the hypothetical ball on the image) but my roommate thinks that the crepes have cooked less in the middle than on the edges, explaining the shape. I'm pretty convinced that with truly uniform crepes the result would be the same. What is your explanation ?

Looking at a lighter flame the other day I wondered whether the inside of the flame was hollow(devoid of flame)? Obviously fuel can only burn with the presence of oxygen this would suggest that just the outside of the flame where it comes in contact with oxygen is actually burning, if so how thick is this 'flame wall'?