1.   No satisfactory way exists to explain how to form a good idea.

2.   Therefore a potential growth stimulatory autocrine cycle may exist to explain the aggressive nature of these cancers.

3.   Actually, though, some other possibilities exist to explain all this.

4.   But to rational minds it has always been troubling that two opposing ways could exist to explain the same universe.

5.   Few clues exist to explain how the flight could have gone so wrong.

6.   Other scientists are doubtful of that explanation, but admit that at the moment no other theory exists to explain the curious finding.

7.   Several theories exist to explain the high number of baseballs leaving major league parks this season.