A node that behaves like 0V without a wire to prove it, and two input pins that act shorted together while drawing no current at all.
Every op-amp circuit-analysis shortcut you were taught — "assume V+ equals V−", "assume no current flows into the inputs" — rests on two ideas that get mangled constantly: the virtual short and the virtual ground. Neither one describes an actual wire. Both describe a consequence of negative feedback acting on an amplifier with enormous open-loop gain. Get the mechanism right and the shortcuts stop feeling like magic and start feeling inevitable.