Ball valves consist of a metal ball and rubber seats. Both attributes were key in the development of the modern toilet, although the materials and the configuration are now different, in the interest of giving less stimulation to the user. Fancy ball valves have O-rings on the stem to prevent leakage, but most use Silly Putty. Ball valves for flammable or toxic gasses have no seals. This promotes leakage, poisoning, and explosion.