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If Stone Age people did indeed use the flight of wild birds as a tool for navigation, could they also have taken their own birds? If these were unable to land on water, e.g. eagles or corvids, when released they would fly ever higher looking for distant land. If they saw land, they would fly …

I have thought of an analogy that best describes the confusing situation in cosmology today. Imagine going on a camping trip with a new tent. You haven't had time to practice erecting it, so you assemble the tent from the rather limited instructions. On completion, you find there are poles sticking out and bits of …

I enjoyed Chanda Prescod-Weinstein's article about a subject that has had us all confused: "What makes a galaxy?" We do have this powerful need to put everything into boxes and, alas, it doesn't always work. For example, is Pluto a planet? This is a subject that seems to cause endless anguish. Another example is the …

I'm not against categorising things, because it helps with understanding, but we need to accept that there will always be things that don't quite fit the model, rather than trying to force them into one box or another. From Sam Edge, Ringwood, Hampshire, UK The debate over whether certain collections of stars should or shouldn't …

In his article "Do aliens do physics?", Daniel Whiteson speculates that "other species might build technologies without developing anything recognisable as physics". Perhaps our fellow terrestrial species that also don't ask "why" demonstrate this using the bedrock of physics: mathematics ( 14 March, p 42 ). For example, honey bees use a "waggle dance" to …

Whiteson proposes that there may be multiple equally valid explanations for the workings of the universe. I have often wondered how differently our scientific outlook would have developed if Benjamin Franklin had picked an alternative convention for the flow of electrical fluid, leading to the electron being regarded as positive and the proton as negative.

Regarding David Robson's piece, "Why are we so suspicious of do-gooders?", in terms of evolutionary theory, altruism increases biological fitness if the cost of altruism is less than the benefit to my relatives, where the currency of cost and benefit is measured in terms of the future spread of my genes in the population. This …

It isn't surprising that an AI might recommend nuclear strikes when faced with the prospect of losing a war regardless of the ethical and moral ramifications. Without a soul, and without the ability to internalise the horror and revulsion resulting from the nuclear attacks on Hiroshima and Nagasaki, of course an AI could consider using …

Of course AIs don't hesitate to use nuclear weapons in simulated war games. A machine doesn't know fear and has no idea what this world is. So it is natural for it to opt for any weapon that is present in the game. It's only following the rules, right?

The piece on hominid hand structure is fascinating and shows the evolutionary importance of the development of hands. Jacob Bronowski, in his book The Ascent of Man, captures this well with the sentence: "The hand is the cutting edge of the mind ( 21 February, p 32)