51.   HIV can also directly attack the brain and central nervous system, although scientists do not know precisely how the AIDS virus produces such damage.

52.   However, biologists say it will be a decade before scientists know whether adult stem cells yield the same potential as human embryo stem cells.

53.   How exactly do scientists know how far away a star is?

54.   If it is significantly more than the original amount, scientists know the mutant form is also present.

55.   If scientists knew the connection, they might be better able to deal with the problems.

56.   If the scientists had known early in their work that Japanese forces rather than German forces might be the first target, would there have been defections?

57.   In fact, scientists know almost nothing about the children who are receiving such drugs, or the circumstances under which the medications are being prescribed.

58.   In the weeks after the earthquake, scientists knew they had something special.

59.   It will be years, he says, before scientists know exactly how these genes lead to obesity and how drugs might block them.

60.   Materials scientists have known that tiny inclusions, or pits, of sulfur-containing impurities, introduced in the manufacturing process, can cause stainless steel to corrode.