What is an easement by prescription and how is it acquired?

Hypo 1: Bob owns Blackacre, a grassy parcel of land with a lake. The lake is full of fish. One day, Amy trespasses on Bob's land and walks from the public road straight to the lake. She sits down and fishes. Every day during the summer, she comes back to fish, parking her car at the edge of the public road, walking to the lake, and fishing. Amy visits so often and frequently that a footpath begins to develop from the road to the lake due to her trampling the grass. For 20 years, Amy continues to visit the lake a few times per week. One day, Bob sees Amy down by his lake and he yells at her, "Hey, this is private property! Get out!" Result: Amy likely has acquired an easement by prescription, allowing her to walk on Bob's land and fish on his lake. This may seem mean, after all, it is Bob's property, but the way the law looks at it is this: For 20 years, Bob allowed Amy to walk on his land and fish in his lake. She did it so often that she created a curious footpath from the road to his lake, which Bob should have noticed if he took care of his land, which would have given him a chance to put up a sign or fence to stop Amy's trespass. Instead, Bob did nothing. For 20 years. As a result, Bob is not now allowed to stop Amy from continuing her use of his land.

Hypo 2: Same facts as Hypo 1, except after 19 years of Amy visiting Bob's lake, Bob runs into her. He introduces himself, and she introduces herself. Bob asks, "What are you doing here?" Amy says, "I'm going to fish in that lake." Bob responds, "Oh, that's my lake. But you know what? You seem nice. You can fish in it. Have fun." After 3 more years, Bob puts a fence around his property, locking Amy out. Result: Amy didn't acquire an easement by prescription, so Bob has the right to lock her out. But why didn't Amy acquire one? After all, this is now 22 years in total of her using the lake. The reason: after 19 years, Bob gave her permission to use the lake, which means Amy's use was no longer adverse, which destroys the element required to establish the easement by prescription.

Fun side fact: Every year during Christmas, Rockefeller Center (the famous place in New York City where they do the large Christmas tree lighting) shuts down their streets to vehicles and allows only pedestrian traffic. One reason they do this is to allow folks to enjoy their tree. The other reason they do this is to exert dominance over their private road. By shutting it down once per year, they destroy any potential easement by prescription claims that may otherwise develop via continuous use of their private road.