Why was the son late for home?

M—Son W—Mother
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 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: On her hands and knees?
M: Yes, in front of her car. She was shocked that she stopped without warning, and the car behind crashed into her.
W: Oh! Was she hurt?
M: No, she wasn’t, fortunately.
W: Where did this happen?
M: Near the church on York Street, where they’re repairing the road.
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 and the other driver got out of his. Then the old lady picked something up, and walked away without saying a word. Then a policeman came; but he didn’t believe their story.
W: Oh?
M: Luckily there was a witness, a man waiting for a bus. He saw it all.
W: I haven’t the faintest idea.
M: She was looking for a false tooth! 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.