If you mean the viral “put an empty plastic bottle in the washing machine” trick, the idea is usually to use a clean, empty plastic bottle as a makeshift agitator to help loosen clothes and improve movement inside the drum.
However, I wouldn’t recommend putting a regular plastic bottle in a washing machine. It can become damaged, make excessive noise, get trapped in clothing, or potentially interfere with the machine. A bottle isn’t designed for the heat, spinning speed, and mechanical forces inside a washer.
For better washing results, use the machine’s recommended load size, don’t overpack the drum, and use an appropriate laundry detergent.
