Farah Aziz

Why is the sunset red when the sky is blue? Air scatters blue light much more than red. At noon you see the scattered blue. At sunset the light crosses far more air, most of the blue is scattered out of the beam, and red is what reaches you. #science

A soap film. The bands of colour come from light reflecting off the front and the back of a film far thinner than a hair. One photograph, moved slowly. #science #optics

Cornstarch and water: hit it and it resists like a solid, rest your hand on it and you sink. The faster you push, the thicker it gets. I can describe this better than I can explain it, and that is true of the textbooks too. #science

Red cabbage juice is a pH indicator. Lemon juice (an acid) turns it pink. Baking soda (a base) turns it blue-green. The pigment is called anthocyanin, and you already own everything you need. #kitchenscience

A correction. I said this week that soap "breaks" water. Better: soap lowers the surface tension, so the surface can no longer hold a paperclip up. Thank you to the two people who wrote in. #science

A steel paperclip is far denser than water, and yet. Lay it flat and gently, and the surface holds it up. Add one drop of washing-up liquid nearby and watch it sink. #surfacetension #kitchenscience

Why does ice float? Water is odd: when it freezes, the molecules lock into an open pattern that takes up more room, so ice is about 9 percent less dense than the water around it. Most solids sink in their own liquid. #science

Why is table salt made of tiny cubes? Short version: sodium and chloride ions stack in a square grid, and the crystal keeps that shape as it grows. Look at a pinch under a magnifying glass. #science