Doors. Devices which, on all civilized planets, remove themselves from the path of people moving from one room to another. Some swish aside, some rush up and down, some dilate, some simply disengage. There are still, alas, backward planets where people must employ their manipulatory appendages to open doors. Such primitive doors, being dangerous disease vectors and traffic choke-points, and reinforcing inequalities with elaborate conventions of who opens for whom, hold back planetary economic and cultural development. When we find such a planet, tragically barred from further progress by its doorknobs, then it is our clear duty to invade and conquer, in order to civilize them.
