If you've ever parked your car at the supermarket and only realised once you got back with a trolley full of shopping that its on a slope you will know what a great idea these are!  Its an acrobatic feat to wedge your foot against the wheel to stop your trolley racing away while putting your groceries or wee one into the car.   Not only are these a fantastic idea, they are so simple to use.  One side has a suction cup and the other a t-like latch that you put through the shopping cart slots.  I was a little worried that they might be hard to get off, or damage the paint on my car but after trying I'm impressed!  Just push on, and lift the little tab before lightly pulling off - you can do it one handed!  What I really liked was that it was small enough to fit into my purse without adding any extra weight.