Fruit produces a chemical called ethylene. As this gas is released it encourages the fruit to make enzymes. The enzymes convert starch to sugar, ripening the fruit and making it more palatable. The fruit becomes softer as the cell walls weaken.
Bananas are usually harvested when they are green, so that when they arrive in the shops they are ripe. Commercial banana firms may apply extra ethylene gas to help ripen the fruit. Apples and bananas make a lot of ethylene. This is why bananas are often stored away from the other fruit.
This experiment allows you to find out the effect of ethylene on some unripe fruit.
Choose some unripe fruit - strawberries or plums work particularly well.
Place two pieces of fruit in each of three paper bags. Add a ripe apple to the first, a ripe banana to the second, and to the third add nothing at all.
Place the bags in different places (as the ethylene produced by the fruit is a gas, it can travel a long way) making sure they are out of the sun.
Dry storage cupboards work well. Leave the fruit for a day and then examine it. The fruit in the bags with the apple or banana is likely to be riper than the fruit left on its own. Leave it for another day and observe again.
Repeat this until all the fruit is over-ripe.
As the fruit ripens it produces more ethylene. So once you have one ripe piece of fruit, soon all the fruit is ripe. The fruit used as a control will not ripen quickly, but as it does ripen the process speeds up as ethylene is produced.