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Debora MacKenzie notes the problem in the economics of new antibiotics ( 19 January, p 20 ). If they are used as sparingly as they should be, not enough of them are sold before patents on them expire for firms to make profits from developing them. I suggest tweaking the current patent system so that …

Is it not a problem that only males were studied – hardly equates to all teenagers? Stella Collins spots a limitation of a study that found those who copy each other's risk-taking have more friends ( 9 February, p 14 )

I picked up the piece by I. Glenn Cohen and Alex Pearlman on medicines that record when they have been taken ( 29 September 2018, p 22 ) at the doctor's surgery. I was more than mildly interested in their use to ensure antibiotics for tuberculosis are taken. In 1949, at 20, I was diagnosed …

You report uncertainty over the UK's supply of medicines in the event of a no-deal Brexit ( 19 January, p 5 ). A family member has glaucoma and uses prescribed eye drops to control it. Without drops, eye pressure rises and puts pressure on the optic nerve. Permanent blindness can result. Is the UK Department …

Sandrine Ceurstemont writes about "water tanks" in Sri Lanka ( 12 January, p 46 ). These are not what people outside the Indian subcontinent usually think of as a tank. An ancient one in Sri Lanka, the Parakrama Samudra, covers 30 square kilometres.

Gisborne, New Zealand Paul Marks mentions suggestions for reducing emissions of carbon dioxide by aircraft, ranging from straighter flight paths to electrification ( 5 January, p 32 ). But he omits the elephantine obvious: reduce the traffic. A globally agreed carbon tax is essential for climate-change mitigation, along with increased consumer awareness.

You praise 10,000 Swedes who have forsworn air travel (Leader, 5 January ). Whether that is a good thing depends on what they do instead. If they go to the same destination by car, with only one or two people in the vehicle, they may be consuming more fuel than if they had taken a …

Guy Cox (Letters, 19 January ) and Eric Kvaalen (Letters, 15 December 2018 ) focus on the strength of the moon and the sun in creating tides large enough to allow marine life to make it onto land. But the sun and the moon together cause the range of tides to vary from "spring" to …

Chris Stokel-Walker mentions Dutch eagles failing to follow orders to hunt drones ( 19 January, p 10 ). Later in the same issue, you again report the intellectual abilities of crows ( p 17 ). It seems that we should try to train crows.

• Before schoolchildren discovered Ediacaran fossils in the UK, geologist Reg Sprigg found some at Ediacara in the Flinders Ranges of South Australia in 1946. These weren't initially recognised as Ediacarans as we now understand them ( 12 January, p 28 ). • Not very heavy: the space rock that hit the moon during its …