The set consists of two plastic, 10cm-diameter hemispheres. Each half has a wooden handle firmly attached to it. One of the hemispheres has a raised edge to hold a rubber seal, while the other hemisphere has a stopcock for attaching to vacuum tubing. Join the hemispheres, hook them up to a vacuum pump, and evacuate the air inside the hemispheres. You will find it hard to pull them apart. Open the stopcock to let air in and the hemispheres will separate easily.