Why was the old lady on her hands and knees in the middle of the road?

M: Hi, mum!
W: Ah, there you are, I was getting worried about you. It’s late.
M: Yes. I bumped into Lyn and went to the pub for a drink. She was telling me about a funny thing that had happened to her yesterday.
W: Oh, what was that?
M: Well, she was driving home from work when suddenly she saw an old lady on her hands and knees in the middle of the road.
W: Oh, her hands and knees in the middle of the road?
M: Yes, in front of her car. She was so shocked that she stopped without warning, and the car behind crashed into hers.
W: Oh! Was she hurt?
M: No, she wasn’t, fortunately.
W: And what was the old lady doing on her hands and knees?
M: I’m just coming to that -- so Lyn got out of her car. Then the old lady picked up something, and walked away without saying a word.
W: Well. I never! She was lucky, Lyn didn’t run her over.
M: Well, yes. Then a policeman came but he didn’t believe the story.
W: Oh?
M: Luckily there was a witness, a man waiting for the bus. He saw it all. Guess what the old lady was doing?
W: I haven’t the faintest idea.
M: She was looking for a false tooth.
W: A false tooth?
M: Yes, it fell out as she was crossing the road. The witness heard her mumbling, "Oh, my gold tooth."
W: I don’t believe it.
M: It’s true. Ask Lyn.