How do tornadoes form?

"The glib answer to that is we do not know. That is why we are studying them. We know the basic processes by which rotation is generated in supercellular thunderstorms. But we do not understand how and why and when these rotations, called mesocyclones, produce more intense, smaller-scale rotations called tornadoes. We do not know how to predict when a particular supercell will produce a tornado. We cannot forecast the size, strength, or lifetime of a tornado. In short, even though major strides have been made in the understanding of tornadoes and tornadogenesis in recent years, we have a long way to go."