1.   But scientists were puzzled by the chains of scalloped, or cycloidal, fracture lines that ran for hundreds of miles across the frozen surface of Europa.

2.   But whatever came before the Cambrian explosion has been an even bigger mystery, puzzling scientists as far back as Darwin.

3.   The case of the warm-water weed now running amok in the cool Mediterranean continues to puzzle scientists.

4.   The finding puzzled other scientists, however.

5.   The reports offered important refinements in the timing of three events that have puzzled scientists trying to reconstruct the mysterious first steps in the emergence of life.

6.   This phenomenon has scientists puzzled about the effects of cigarettes on teenagers.

7.   A natural chemical that scrubs many manmade pollutants from the atmosphere has gone into a sharp decline, puzzling scientists who have studied the chemical for decades.

8.   Frog deformity puzzles scientists.

9.   Remote-controlled garage door openers puzzled the scientists for a while.