What does Rebecca Nurse think is wrong with the girls in act one?
In the first act of The Crucible the details behind the girls illness are varied and not very detailed. The audience knows that the girls were caught dancing in the woods, and that (from Abagail's own admissions) that Betty's not sick but only "gone silly somehow."
Rebecca Nurse (who knows about children) has seen children go through all different sorts of fads. She believes that the girls are just going through one of these times in their lives and will stop as soon as they get tired of it.
Rebecca sitting. I think she'll wake in time. Pray calm yourselves. I have eleven children and have been twenty-six times a grandma, and I have seen them all through their silly season, and when it come on them they will run the Devil bowlegged keeping up with their mischief. I think she will wake when she tires of it. A child's spirit is like a child, you can never catch it by running after it; you must stand still, and, for love it will soon itself come back.
Rebecca, the calm voice of reason in the town, is disappointed that her neighbors are so quick to blame witchcraft as the cause behind the girls illness. She knows so many people (including Parris) believe children should "be seen and not heard" and she thinks the children are just rebelling against this in their own way.
Rebecca Nurse is somewhat of an expert on children, having had several children of her own and then, by her own count, twenty-six grandchildren. Rebecca seems certain, from the outset, that Betty's apparent illness has nothing to do with witchcraft (even without the benefit of Abigail telling her as Abigail told John). She says, "A child's spirit is like a child, you can never catch it by running after it; you must stand still, and, for love, it will soon itself come back." She implies that there is nothing whatsoever wrong with Betty and Ruth. She believes that they are acting like typical children: somewhat willful and a little silly. When adults try to force them to do something, they resist. Rebecca thinks, and she is right, that the supernatural has no part in this, and that the girls are just engaging in a bit of childhood mischief. She doesn't judge the girls; she just seems to feel that this is typical behavior for children. This really shows us her good judgment and wisdom right away.