They're very light to the touch, allowing them to be pushed firmly onto the plastic pieces, which are attached to rounded metal legs at each corner. The only problem is that the tapping direction of these round pieces is upward, not downward. The only way to successfully assemble these shelves is to turn them upside down, with the metal ledges sitting on top of the shelves. I've built dozens of devices in my life and have never had this problem. I need these shelves and keep them to avoid the hassle of returns. Rip off.