Read the City Walking Tours

I met the business owner in Marin, and was impressed by her unique description of the tour. This is not your grandfather’s walking tour of SF!
We met Mary (owner/​guide) near our office and got a 15 minute introduction of what to expect on our walk and safety tips («we won’t cross unless the walk-​man is lit up white!»). We ended up walking for about 2 ½ hours, covered about 2 miles, and learned a TON. Questions that i can now answer:
1. Why are there small, nickel sized brass coins nailed to the sidewalk, and what are they used for?
2. Why do real estate developers dislike the commonly used ficus tree in front of their buildings?
3. Which secret tunnel in Jackson Square used to transport workers that got «Shanghaied?»
4. Why was there a foot bridge over the pond that used to be near the Trans-​America Building?
My only wish would have been a scheduled stop into a historic bar at the end, but we ended up doing that anyway!