The increasingly rarer moment when you spot someone a block away talking out loud to no one and are unable to tell whether they are using a Bluetooth device or having an argument with the voices in their head.
A space between two buildings that is too narrow for a human to enter and is therefore off limits to our species for decades, perhaps centuries.
The shrill siren noise big trucks make when they back up.
The relationship between radically different businesses that have nothing in common besides the fact that they occupy the same building. Shared HVACs, bathrooms, and loading docks, joint fire drills, employees who know the name of the property management company–this is all vortelation. China Miéville wrote a whole book about it.