In Act I, Betty reveals that Abigail drank blood as part of her charm to kill Elizabeth Proctor. While the girls are in the upper room alone, they talk freely about what happened in the woods out of a sense of real fear regarding what they did in the woods.
"Betty: You drank blood Abby! You didn't tell him that." (Miller)
"Abigail: Betty, you never say that again! You will never." (Miller)
"Betty: You did, you did! You drank a charm to kill John Proctor's wife! You drank a charm to kill Goody Proctor." (Miller)
In Act I of "The Crucible" Betty does not say a word because she is pretending to be bewitched. However, once all of the adults have left the room and Betty is left with only the other girls, she does reveal something about what happened in the woods. She says, "You drank blood, Abby. You didn't tell him that!" Therefore, she reveals that Abigail drank blood in the woods; further reading suggests that this was a charm to kill Goody Proctor so that she could have John Proctor to herself.