Time.

It is often said that 'time is the fourth dimension.' This is consistent with our definition, since to meet someone you should specify a time as well as, say, the crossroads (or lattitude and longitude) and the floor (if you'll be meeting in a building).

This isn't the most precise explanation of dimension, since we often give an arbitrary amount of information (e.g., Mallot hall, corner of tower and garden, 5th floor, room 501, right beyond the stairs, 10 o'clock, Monday). We do, however, consider time to move linearly which adds a dimension. To see this clearly we might go back to 1-D (1-dimensional) world and consider the added dimension of time as an added spacial dimension.

If we let the vertical direction represent time, we get the following plane as our 'space-time'.



Can you tell what this figure represents to the 1-D worms?