Many birds seem to take long, hopping journeys across grass or other areas by hopping erratically. Is this not very energy-consuming, compared to walking, or flying?

What might they gain by this form of motion?

None of the countless species of animal in existence has three legs. Creatures such as the kangaroo and the meerkat use their tails for balance, but a tail is plainly not the same as a leg. This pattern does not apply only to mammals - other kinds of animal have an even number of legs, too. Why wouldn't having three legs work?

Monika Hofman, London, UK

Obviously on a flat road cycling is a more efficient form of transport than walking but when faced with a steep incline I would prefer to walk (despite being a competant cyclist), but surely the mechanisms which make cycling preferable on a shallow incline are still relevent. Am I misguided in my assuption that walking is more efficient on the steep? If not then at what incline does the swich over happen?

While walking in the Cairngorms, a path had muddy puddles one day, and was frozen the following day. Parts of the path were the mixture of ice, air and gravel pictured. The structures are noticeable, because they collapse when stood on. They are different to any normal frozen puddles, because they are not solid ice, nor are they a layer of ice with water or air beneath. The pictured piece measures about 65x65 mm.

A pathway in my neighbourhood splits at an incline into both steps and a slope. Which option requires more calories to walk up or down?

Yonatan Silver, Jerusalem, Israel

(Image: Pat-swan, stock.xchng)