What is the most useless invention that exists?

I think there are some who fight for that title, but as far as I’m concerned, the Pet Rock should be #1. They were ordinary gray stones bought from building supply stores and sold as live pets in cardboard boxes designed for this purpose, the inside of the box had a straw padding and the box had holes for the ‘pet’ to breathe. The idea came from Gary Dahl, an advertising executive who sold 1.5 million of these ‘pets’ in 1975 and became a millionaire.

Included was a 32 page manual titled The Care and Training of Your Pet Rock which contained instructions on how to care for the pet rock. The instruction manual was the actual product and full of jokes and puns; it contained many commands that could be taught to the new pet. While the commands ‘sit!’ and ‘quiet!’ were easy to comply with, the command ‘roll!’ usually required a little help from the trainer.

There are others I can mention:

Lateral position bed : many of us like to sleep in a crouched lateral position, but this invention takes it to the extreme. Be careful not to fall.

Mini golf in the bathroom: Boredom in the toilet is over, you can now put aside the magazines and the cell phone.

Watermelon cart: it is true that watermelon can be heavy and uncomfortable to transport, but… Will a specific cart be needed?