(I) Einstein, who is 21 in 1900, has been puzzling about
visible light for five years, which does not always follow
known rules. (II) He has not yet been able to figure out
what will happen if he travels on a beam of light. (III) But
he already knows that there are particles smaller than
atoms. (IV) Now, the idea that a blackbody changes
energy by discrete amounts during the emission
process gives Einstein something else to consider.
(V) Then, Einstein has to describe the invention and
give the reason for why he invented such a device.