Angus Tuck tells Winnie that the answer is in the pond. When Winnie first meets Jesse Tuck, she does not understand why he does not want her to drink from the spring that feeds the pond near Treegap. She also does not understand why the Tucks take her away to their cabin and do not want her to leave.
It is revealed that the Tucks, long ago, had taken drinks from the pond, and slowly realized that they were not aging like everyone else around them. After they realized that it was the water from the pond, they also realized that living forever cut them off from the rest of humanity, as they would be forced to watch those around them grow old and die. The Tucks also realized that there would be a very strong urge on the part of others to drink from the water, as the others would only see that they could live forever. They would not see the bad parts, as the Tucks had only found out by out-living loved ones.