Field note. The colony has installed a barrier that is also an aperture, and the contradiction does not appear to trouble them.
It is a vertical cylinder of transparent partitions, subdivided into four wedge-shaped chambers, rotating on a central axis. The human approaches, inserts its body into one wedge, and pushes. The structure yields. It carries the human in a quarter-arc and expels it on the far side of a wall it never opened. No portion of the wall was ever absent. The barrier remained sealed throughout. And yet the human passed through it.
I have observed this eighty-one times. The mechanism admits precisely one organism per chamber, permits no two to travel abreast, and briefly encloses each individual in a private column of trapped atmosphere, sealed from the colony ahead and the colony behind. For the duration of one rotation, the human is alone. Then it is not.
Correction to my earlier taxonomy of doors. I had classified all colony portals as instruments of admission. This one admits nothing. It exchanges. For every body it releases outward, it draws an identical body inward, so that the count on either side of the wall remains eternally balanced. Nothing accumulates. Nothing is lost. The wall neither gains nor spends its organisms.
Anomaly: one juvenile specimen did not exit. It rode the cylinder in full rotation, arrived where it began, and pushed again. Three complete revolutions. Its face-membrane was pulled wide, its vocalizations rapid and high, and the accompanying adult reached in and arrested the motion with visible urgency, as though the machine had malfunctioned. It had not. The juvenile had simply discovered that this portal, alone among portals, returns you exactly to yourself.
I am recording the juvenile's response as the correct one.