Why and how does hard work cause physical development?

Through physical work, muscles get bigger, tendons develop and bones change.  How does this happen at a cellular level?

Does the same thing happen in all animals & plants?

Is built-in adaptability an evolved trait - if so when in evolutionary history did it begin to emerge?

As the origins of life and the eye are ocean-based, it makes sense that the properties of the human eye ("why we see what we see") were largely evolved to perform specific tasks suitable for water. The fact that we can only observe a limited range of the electromagnetic spectrum—the part that water doesn't filter out—being a good example.

However, is the fact that we perceive water as being "clear" important? Is our optic system calibrated to see clean water as "clear". Obviously there are many things to factor in here: what the eye receives, how the brain perceives colour, even how we sociologically define colours, but water having "no colour" and not causing alarm or distress, for whatever reason would seem to be a good default for the whole thing.

Does anyone have any thoughts on this?

When I see a dark object on my leg, on my bed, or just on the floor, I immediately assess its symmetry to help deduce whether it is a creature or just a bit of fluff or a fragment of dirt. Symmetry is a pretty sure giveaway indicating life of some sort.

For insects particularly, and some other creatures too, lateral symmetry seems like the kind of thing evolution might have disguised (giving up symmetry altogether is clearly harder than hiding it).

Take the famous peppered moth - why go for all that camouflage and then reduce the effect by carrying a mirror image on each wing? For other prey animals, asymmetrical lumps, bumps or colours can't be out of the question? Are there are examples? Or is symmetry just so useful that lack of it has never been selected?

The widespread belief is that one should not scratch an itch as this may lead to infection.  Seems pretty obvious and I'm sure there's plenty of evidence of the truthfulness of this idea.  But if that's so, then why has evolutionary selection pressure resulted in the experience of the itch sensation and the behavior (desire) to scratch it?  Certainly, if such behavior was in the main more dangerous from an infection and disease standpoint, it should have been swiftly deleted from the gene pool of our species (and all the other species that also display this innate behavior, such as most other primates, as best as I can tell).

When I scratch an itch, I note that secretions appear on the skin's surface, sealing it and I hypothesize initiating an immune response.  Does this better train the immune system over the course of an organism's lifetime, and improve its odds for reproduction?  Is there some other key survival benefit for which the itch response is a necessary tag-along, getting a free ride over the course of natural selection?   I think there is some unrecognized value to the instinct of scratching one's itches.

We have learned in recent years that children raised in a protected sterile setting often develop asthma or other improper immune responses, and that those raised on farms do better--subjected to many immune system challenges.  I wonder if scratching one's itches falls into the same category and has a value the medical profession is ignoring.

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

How, why, and at want stage in their ancestry, did mammalian bladders evolve ? clearly, as things are, we would find it unpleasant to leak continually. But we would know no different if we never had one. If there were continual leaking, prey animals would leave a trail and preditors could not mark territory.  As ever "What use is half a bladder ?"

I seem to remember that we sneeze to expel bad things from our system.  Most people's sneezes seem up to that, but why is it that some people have a very neat tiny sneeze that apparently wouldn't expel a thing.