To the 140th (2016) and the 115th (2017) birthdays of the famous German chemists
Otto Diels and Kurt Alder:
This is a model of the endo form of the dimer which results from a Diels-Alder reaction of two molecules
cyclopentadiene. If you take a closer look, you can see the two new C-C bonds forming
(or disconnecting in a retro Diels-Alder reaction).
This model version does not show the hydrogen atoms as usual in organic chemistry.