  Enrico Fermi estimated the yield of the Trinity test to be about 10 kilotons
by dropping small pieces of paper and observing their motion in the blast wave.
This is about 40% of the radiochemically derived value of $25 \pm 2$ kilotons
that necessarily includes thermal and nuclear radiation that do not contribute
to the blast. Although this story is classic, there appears to be no account of
how he related his observation to the yield. This note attempts to reconstruct
how he might have done so.
