Welcome to this Friday's round-up of the week's top questions.

Ever been puzzled how you can blow both cold and hot air out of your mouth? purple2410 asked The Last Word, and got some very interesting replies.

A discussion on whether a body will use more calories and water if there is more available to it branched into the effect of weight on the speed of freewheeling downhill on a bicycle.

Here's one that has always puzzled me: how do fish get up to the top of a mountain?

Last Worder Milon has some ducks that are behaving strangely… they seem to want to play with their own version of a rubber duck. Can you explain why?

Finally, today's question gets to the lungs of the matter: what causes the burning feeling you get in your throat and lungs when you exercise beyond your normal capacity?

All the best,

Kat

Letters and comments editor, New Scientist

Some time ago I was riding old bicycle when the chain broke. I managed to roll 10 kilometres (150 metres down) on a former railroad trail so I got almost to my point of destination without having to get off and push. But it was very slow. I think the weight of my bag helped me. There were tools in it (unfortunately, none to repair a chain) but also 1,5 litres of water and some apples and sandwiches. I was afraid to comsume any during the rolling part not only because I was unwilling to stop the mostion but also because I thought it might reduce the weight. Would it? There was no toilet on the way, so the question is: wuold my body have breathed out more CO2 and breathed and sweated out more water if it had got more? I was not critically hungry or thirsty at that time, just a bit.

When I lift my wife for a good hug, her weight seems vastly different depending on whether she's limp or rigid. Is there anything real about the difference or is it my imagination that she seems much lighter (or certainly much easier to lift) when she's rigid? I noticed the same thing a few years back when I used to carry my kids to bed.

Geoff Patton, Wheaton, Maryland, US

I notice that at random times my stomach rumbles unexpectedly... what is actually going on down there and why is it happening?

Aside from bone and muscle differences, which have already been covered.

For example, someone who grows up without being spun around at 400m/s as the world spins; would they feel as if they were spinning when they set down or entered orbit?

In common parlance, one would say the body 'burns' food, and, indeed, we are warm creatures due to some kind of exothermic reaction(s) inside us, yet our muscles and brain work on electrical impulse. Is this the most efficient conversion of heat to useful energy we know of, or is the electrical energy converted from chemical potential energy somewhere?