Ethylene
Ethylene is a gas that fruits like apples release to signal nearby fruits to ripen together.
Think of it this way
When one banana in a fruit bowl starts to ripen, it gives off a gas that drifts to the other bananas nearby. That gas acts like a signal. Each banana that gets the message starts to ripen too. Soon the whole bowl ripens at once, even though nothing touched them.
Explaining ethylene by grade level
Apples make a gas you can not see or smell. This gas tells other fruits near it to get ripe. Put one ripe apple next to green ones in a bag. They all turn ripe at the same time.