This black magic reaction is very funny, as the reaction is so quick you can see the colourless water will sudden turn black.
The following two solution are prepared separately and then mixed it together.
Solution A: 0.38g sodium metabisulphite + 0.6g starch to 80ml water
Solution B: 1.4 g potassium iodate to 200ml water.
As starch and potassium iodate is difficult to dissolved in water, it is suggested to use hot water instead of cold water. After mixing two solutions, it will turn to black after 13s.
1. More the potassium iodate, faster reaction to turn black
2. It is suggested to clean the breaker at once after the experiment
3. There is another recipe using potassium iodide instead potassium iodate, but the procedure is more complex and more difficult to control the reaction timing