Fermi problem game thread

I thought it might be fun to make a little game out of asking Fermi problems and trying to work out solutions to others'.

What is a Fermi problem?

A Fermi problem is (to quote wikipedia ):

an estimation problem designed to teach dimensional analysis or approximation, and such a problem is usually a back-of-the-envelope calculation. The estimation technique is named after physicist Enrico Fermi as he was known for his ability to make good approximate calculations with little or no actual data. Fermi problems typically involve making justified guesses about quantities and their variance or lower and upper bounds.

Basically, these are questions that would be very difficult to calculate exactly without looking up the answer. The goal is to arrive at a good estimate by making justified assumptions. As such, looking up facts and statistics should be minimized or outright avoided (e.g., if the question is "What is the mass of the water in Lake Michigan?", you shouldn't look up Lake Michigan's volume to aid your answer. This should be estimated from things you know off the top of your head).

The way this thread works

1. If you have a Fermi problem, post it as a top level comment.
2. If you have a solution to an already posted Fermi problem, post it as a reply to that comment. Be sure to post your reasoning and thought process.

Fermi problems can be fun to come up with, and fun to answer. Examples of Fermi problems might include:

• How many piano tuners are there in Chicago? (this one is apparently one that Fermi came up with himself)

• How many keystrokes occur worldwide, daily?

• What is the mass of the water in Lake Michigan?

• How many cars are there in New York City on a given day?

• How many blades of grass are there on the National Mall?

• How many eggs are consumed in the US each day?

Since many of these have no definitive answer, it is not always possible to score the 'correctness' of an answer. Nonetheless, others can judge when an answer makes reasonable assumptions, so feel free reply to answers with suggestions of which assumptions can be refined or further justified.