Two mathematicians have proved that a straightforward question—how hard is it to untie a knot?—has a complicated answer.

E-bikes, e-scooters and mopeds have become some people's main way of getting around in dense urban centers where driving or ...

Every animal and plant needs food to live, and they are all part of a food chain. A food chain shows how animals obtain their food. A food chain also shows the flow of energy, which starts off at the ...