Note however,
that this rule is not always so clear, even to professional Physicists.
The meaning of this picture is somewhat different; it deals with bosons,
like for example photons, but also they should occupy the lowest states.
"Bosons-photons (birds) occupy the steps of ladder of quantum oscillator.
Numbers 0,1,2,3 correspond to the excitation level. At the lowest level
there is
no birds, but the energy is not equal to zero".
[A. Góral, Meandry Fizyki, MON, Warszawa]
