What causes double rainbows?

What is deja vu? Why do certain magnets attract? Why do tea stains form in a ring? Can you scientifically work out someone's date of birth? How does mercury and lead stay in your body? Do mermaids exist? Do helium balloons defy gravity? What causes a double rainbows? Why does oxidised cholesterol cause heart attacks? Plus the life saving backpack for bees! Like this podcast? Please help us by supporting the Naked Scientists