At some point, everyone has transposed the first letters in two words and came up with a nonsense phrase. You might mean to say "barn door," but it comes out "darn boor."
These slips of the tongue are called spoonerisms. Cognitive psychologists study them because of what they say about how our brains construct language.
One At A Time... Or Clumps?
Early twentieth-century psychologists believed that language was produced in our brains one word at a time, that each word acted as a stimulus to produce another word. But cognitive psychologists now believe that we produce language in clumps rather than one word at a time.
The study of spoonerisms has helped scientists formulate these new theories. Spoonerisms may seem like random mistakes, but, in fact, they follow a regular set of rules. When two sounds are transposed between two words, they are almost always sounds that belong in the same positions.
For example, the beginning of one word almost never exchanges with the end of another. The close association your brain makes between two words such as "barn" and "door" indicates that your brain chose those words as a unit, rather than one at a time.
We make speech errors like this because as we construct language, our brain builds a frame for what we are going to say before we choose the actual words that will go into that frame.
When we get a phrase right, our brains have successfully coordinated this frame with the sound of a word. Spoonerisms happen when this coordination breaks down, often because of the interference of external or internal stimulus.