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?

I keep two PET bottles filled with tap water. As I don't intend to use the water unboiled anyway, I hardly ever refill them. Mostly I use them as weights for some fitness training (like dumbells). But once when the water was turned off I actually used water from one of the bottles and refilled it afterwards. Now in comparison with this newly-filled bottle I see that the old one has lost some water. No air has come in instead of it, but the bottle has become a bit floppy. I understand that water molecules are smaller than most molecules in the air, so it is understandable there might be pores through which it can evaporate and nothing comes in to replace it. But are these pores in the bottle or lid itself or does the vapour go through between bottle and lid?

Is it better to boil water in a kettle or on a gas hob. Which is better for preventing global warming?

What is it in a fountain that allows the water to come surging back up through the tube? What technical device (or natural device) enables the water to be forced upwards and how does it work?

If I keep a plastic mineral-water bottle topped up with tap water and regularly drink directly from it, the neck smells vile after a couple of weeks. Why is this and why is it always exactly the same smell?

Ann Gilmour, Belfast, UK

My colleagues and I were discussing what physical changes the flow of water goes through between leaving the tap and hitting the sink. This was noticed by feeling greater pressure when an object is held at the bottom of the stream versus closer to the tap.

Given that the volume of water flowing from the tap is a constant and that the water is subject to acceleration under gravity during it's fall, presumably the water stream is decreasing in diameter to maintain the same flow rate?

Presumably interference with the air would a place an upper limit on the speed at which the stream would remain together but what would this be and what would the upper limit be in a vacuum?

My daughter dived underwater in the swimming pool and screamed as loud as she could. I was right next to her with my head out of the water, but I could only detect the tiniest sound, at the end of the scream. But when I was underwater with her, I could hear most of the scream. Why?

Linda Simpson Katonah, New York, US