HOMO of Cyclopentadiene	Keyboard Commands for controling the models   Arrangement that leads to endo product.	endo-5-norbornene-2,3-dicarboxylic anhydride
LUMO of Maleic anhydride	Arrangement that leads to exo product.	exo-5-norbornene-2,3-dicarboxylic anhydride

Lining up the models on the left so that they resemble the arrangements (the models in the center) that lead to the endo or to the exo product reveals that there is better orbital overlap between the HOMO and LUMO when forming the endo product. Better interaction between the orbitals implies a lower energy transition state. Thus, the endo product will be produced in excess when the reaction is under kinetic control.