Let's trace the Diels–Alder reaction. Step 1: Take 1,3-butadiene (the diene) and ethene (the dienophile). Step 2: Imagine the electrons dancing. The double bonds shift to form two new single bonds and one new central double bond. Step 3: These two molecules fuse together. Result: You get cyclohexene, a 6-carbon ring. It is like building a box from two flat sheets of paper.