Science pack

The sky is blue because of Rayleigh scattering. Air molecules scatter short wavelengths far more strongly than long ones — blue light is scattered roughly 5.5 times more than red — so blue light bounces around the atmosphere and reaches your eye from every direction. At sunset the light travels through much more air, scattering the blue away and leaving red.

A battery converts chemical energy into electrical energy through a controlled reaction. Two electrodes sit in an electrolyte that lets ions pass but blocks electrons, so the electrons are forced to travel through your device instead — and that flow is the current. Batteries degrade because the reaction is never perfectly reversible.

A vaccine trains your immune system to recognise a pathogen without you having to catch it. It introduces an antigen — a distinctive molecular shape from the pathogen's surface — so your body mounts a full immune response and then stores it as memory cells. If the real pathogen arrives later, those memory cells recognise it and destroy it before it can make you seriously ill.