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You are right to highlight the importance of heat storage as a way to use surplus renewable electricity. A big but under-appreciated resource on this front is the hot water tank. We have about 9 million in UK homes, typically supplied from a gas boiler, with a 3-kilowatt immersion heater backup ( Leader, 20 July …

Some things don't change. Our ancient ancestors valued caves as a haven from the weather and wild animals. Now, we are considering using caverns on the moon as protection from space "weather". Perhaps we should think about installing a copy of the Lascaux cave paintings in the first lunar cavern we use for habitation ( …

As a scientist, I prefer to think in terms of components, causality and processes. It would be good to know more about these when it comes to ultra-processed foods ( 13 July, p 10 ). There are many questions. Which additives might degrade the mucus lining of my gut, leaving me at risk of bowel …

In your look at the possibility of extra dimensions, Georges Obied says: "There's no reason why it has to be three. It could have been two; it could have been four or 10 ( 13 July, p 32 )." I wonder whether we can look to the anthropic principle as to why we have at …

Andrew Whiteley notes that the idea of our cosmic insignificance involves "equating physical size with significance", which he rightly decries as absurd. A similar point was made by the polymath Frank Ramsey: "I don't feel the least humble before the vastness of the heavens. The stars may be large, but they cannot think or love; …

Geshuang Chen's photo shows an optical corona ( 13 July, p 25 ). To gauge the ideal bowl size for spent pistachio shells, they were modelled as hollow half-spheres with 73 per cent of the packing density of the whole nuts in shells ( 13 July, p 9 ). Boston Dynamics's robots are banned from …

You report evidence of an earlier origin of LUCA (the last universal common ancestor of all life), around 4.2 billion years ago. If LUCA emerged then, this would give just a 300-million-year window between the formation of Earth's surface and life's origin ( 20 July, p 9 ). It seems unlikely this could have occurred …

Your article on psychological barriers to exercise only addressed one aspect of the issue. I doubt I am alone in associating exercise with fear, even when someone else is doing it. I had to take my courage in both hands to read all your articles on exercise, and by the end I had definite signs …

Gravitricity's energy-storage scheme uses a 25-tonne weight dangled in an old mine. Since a great deal of wind energy is generated offshore, we could use excess electricity to pull a large air-filled float down to the sea floor and, in times of need, use its rise to generate electricity ( 20 July, p 36 ). …

Roger Harrabin speaks of massive energy-storage projects involving weights in deep mines, but it occurred to me that all new-build houses could include some sort of double wall with an individual "falling weight" device, wound up by the output of solar panels or wind generators during daytime and allowed to gradually fall in the night, …