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Every atom has a fixed number of hands, and every single hand has to end up holding another one.
Drag from an atom to another atom to join them up. Drag between the same two again and they hold on twice — that is how a double bond works. Tap the little − in the middle of a join to let go again.
Get every hand holding, in one piece, in the right shape, and you have built a real molecule.
Computer: R starts the one you are on over, ESC goes back to the packs.