Dr Chris and Dr Helen answer all your burning science questions, including why frost can form even when the air temperature is above zero, why hair looks darker when it is wet, why sunlight looks red through your eyelids, and whether cracking your knuckles really causes arthritis. We also talk to Chemistry World editor Mark Peplow about venomous vipers, artificial kidneys, and how LSD might be switching on hallucinations, and in Kitchen Science Anna Lacey and Dave Ansell look at some slightly safer visual effects with the help of a sodium street light.