Sensational - the try 'em at home guide to illusions

The Sweetness Illusion

The way we perceive the sweetness of sugar depends on its temperature. So try this. Take some sugar and water and mix in a bowl, then pour half the liquid into one glass and pop this glass in the fridge to cool. Take the remainder and pour it into another glass and leave it near a radiator to warm up. The science says that the perceived sweetness of sucrose (sugar) increase by 40% as the temperature increases from 4 degrees C (about fridge temperature) to 36 degrees C (about body temperature). Take the two glasses of liquid and ask a friend which tastes sweeter (of course they both have the same amount of sugar in them). If your friend says the warm glass is sweeter then you have your illusion.

If you want to be really scientific about it, make more liquid in the bowl and put it equally into four glasses. Put one in the fridge, one by the radiator and leave the other two in the room. The two in the room are called the experimental control. Ask your friend first to taste the sweetness of the two room temperature glasses. Because they have the same amount of sugar and the same temperature they should taste the same sweetness, so our control will show that it is in fact the temperature that's causing the effect and not, for example, that the first glass drunk always tastes sweeter.

It's also a good idea to have your friend swoosh their mouth out with normal water between tastes of the sugary water so you don't contaminate your samples.