When I lie down, I like to sleep on my back.
Here"lie" is a transitive or intransitive verb.

To lie is an intransitive verb:

Explanation:

It shows action, and the subject of the sentence engages in that action, but nothing is being acted upon (the verb has no direct object). Put another way, the verb to lie does not express the kind of action that can be done to anything. Remember that it means “to recline” or “to rest.”