1.   But the experts say it is still a mystery as to what caused so many tornadoes and why a few of them were of such width and strength.

2.   Gaining strength as it moved east, the storm brought heavy winds and rain to Illinois late Thursday, causing more tornadoes and spreading severe damage.

3.   Q. What is it about this region that causes tornadoes?

4.   The classic example, called the butterfly effect, asks whether a butterfly flapping its wings in Brazil could cause a tornado in Texas.

5.   Tornadoes are caused by strong thunderstorms which derive their strength from humid air and energetic jet stream disturbances.