As fruit ripens it give off a gas. This gas speeds the ripening process. By placing the fruit in a bag or paper you capture and concentrate that gas and speed the process.
Other fruits will not be damaged if placed in a bag with a banana. However, other fruits will ripen faster when placed in a paper bag with a banana. Some may ripen so fast that they spoil.
You place them on the windowsill.
Avocados are hard when they are not yet ripe. To ripen it quickly, place the avocado in a paper bag with a banana or apple. The ethylene gas produced by these fruits will help speed up the ripening process.
some fruits can ripen when exposed to high levels of CO2 or etilen which is a plant hormone (bet my spelling is wrong but that's how you say it).
Pineapple
They give off a lot of ethylene gas and even make other fruits and vegetables stored with them rot or ripen quickly.
Frequently is used the ethylene oxide.
Pears ripen pretty quickly, but if you put in a bag with a banana or an apple the gases from these fruits will make the pear ripen quicker.
I think you mean an apple will ripen bananas or green tomatoes. The apples as they ripen give off gas that aids and speeds up the ripening of other fruits, like placed in a brown bag with bananas or tomatoes and other fruits. Apples stored in the frige crisper drawer will make other fruits ripen quickly, thus go bad before you use them too.
Store them in a paper bag and they will ripen. Fruit gives of a gas that helps it ripen, it is a very small amount the paper bag helps to concentrate it.
Bananas ripen quickly on their own as they're one of the few fruits that release ethene. Keeping them together in a plastic bag should make them ripen. Keep them away from other fruit that you don't want to go off, and if you want avocados to ripen put them in a brown paper bag with bananas in an airing cupboard.
To ripen a mango at home, place it in a paper bag with a banana or apple. The ethylene gas produced by these fruits will help speed up the ripening process. Keep the bag at room temperature and check the mango daily until it reaches your desired ripeness.