# 14.
My humble addition.
If you feel you have an answer to this puzzle, feel free to supply one.
The Moon has been around for 4 plus billion years.
The Moon's surface has been little disturbed over a billion years;
a Copernicus Crater and a few other big ones not withstanding.
And the same lunar surface has been bombarded by cosmic rays
for billions of years.
You would expect those billions of years of cosmic radiation to have left the surface of the moon dangerously radioactive. [prior to 1966, nearly all astronomers and physicists believed that the moon's surface was intensely radioactive as a result.]
So why isn't the Moon's surface highly radioactive?
Do we have a #14?