1. Walk through the empty rooms, but cautiously. It may be that the inhabitants have gone to some better place. If so you should be able to see the discarded ticket stubs lying around in those places that people normally put small things. In this case the world is safe to explore further, although you should always make sure that your door is propped open.

2. Some empty worlds, of course, are radioactive or poisonous. There should be a geiger counter in the rucksack. Always check that the inspection sticker is up to date. It is unusual for empty worlds to be empty of all life, so you may want to check for small animals or insects that have been poisoned. Leave these worlds with all the speed you can. Do not pause to take photographs.

3. It may be that you have entered the memory-world of some person who prefers buildings to people. Memory-worlds can be distinguished by the lack of detail between the main features. Try looking around the back of major landmarks, inside cupboards, under inspection hatches or in the room at the end of the assembly line that appears to be locked but in reality is not. An abundance of blank grey or white spaces is a giveaway. You should leave these spaces before you are forgotten. However they may be useful in revealing details of their progenitor that would be otherwise difficult to obtain.

4. Alternatively you may have stumbled across a simulation of a world before the latter stage in which it is populated. If you have entered during some kind of debugging phase, you may be able to discern the difference by finding bugs. Try interacting with your environment in a testing fashion, for example dropping and throwing objects, pushing on walls and corners, or putting things in water. You may also be able to uncover the save button. It is often located under a doormat or behind a large family portrait.

5. An empty world may be a dream that you are having. If you have taken a door into your own dream, it will feel oddly familiar. There is a danger of circular reasoning, particularly with the chance that you may encounter your dreaming self on the way out. Although it is tempting not to, you should leave these worlds at once.

6. The world may be filled with people, but they are half a second offset from your position in time, so that you are unable to see them. These worlds are safe, but you will function in them as a time-displaced poltergeist, causing havoc that you cannot see. If this idea appeals, by all means stay. You can tell these worlds by the warm coffee left on tables, by laptops left out with unsaved work, and by shopping carts left in the aisles. Some people find that eating food at a time offset to your stomach causes indigestion. This is the primary hazard you will face from an extended stay here.

7. A world may be empty if its original inhabitants, fearing some oncoming catastrophe, have evacuated it. In this case the buildings may be neatly shut up, valuables will have been decanted from their boxes, and shoes will have been taken from out of shoe racks. In some cases a single flower will have been left to wilt at each doorstep. Depending on the nature and realism of the disaster, you may wish to return through your door at once. But these worlds may also serve as useful guides to the sort of catastrophes that may one day afflict your world too, so it can be useful to stay long enough to read those newsletters that have been discarded in street bins or on restaurant tables.

8. An empty world may have budded up out of stuff as a trap for travellers. This sort of world will fold up around you while you sleep and digest you slowly. These worlds may be discerened by the alien organicity of their small places. Unusual fractal patterns, curves where angles are expected, or angles where curves are expected are all clues that you should watch out for. You may continue to explore these worlds if you wish, although there is little enlightenment to be gained there. Obviously do not sleep.

9. Finally, a world may be transiently empty. For example, it may be your own world, but the inhabitants you were expecting may be on holiday. If all other indicators have turned up negative, try travelling further afield. The tide of people may have washed to some further hill, ready to wash back at any moment, free from imagined disaster and ready to start their days anew.