In Civilization 4 how do you combine units?

In Civilization IV, you can combine units by moving them into the same tile. When two or more units of the same type (e.g., two archers) are stacked together, they will automatically combine into a single, stronger unit. However, you cannot merge different unit types or combine units that are not of the same category (e.g., melee with ranged). Additionally, if you combine a unit with a higher experience level with one of lower experience, the resulting unit retains the higher experience level.