Enrico Fermi walked into interviews and produced piano-tuner counts out of thin air: chain a handful of round numbers together and the answer lands within a factor of ten of the truth. Pick a classic question or build your own chain — the running product, a powers-of-ten ruler and the real-world sanity check update as you type.
First row is the starting quantity; every row after it multiplies (×) or divides (÷) something you can guess to one significant figure. Rounding each factor to a friendly number is the technique, not a bug.
Each dot is one step of the chain on a log₁₀ ruler — the whole point of Fermi maths is that you only ever need the exponent to be right.
Compounding rule of thumb: if every factor is only accurate to a factor of 2, a five-factor chain is “accurate” to ×/÷ 32. That is why Fermi answers are judged as decades, not decimals — an estimate inside one order of magnitude is a success.