The Lay? Lie? of the Land
Lay and lie (in the sense of "to lie down") are confusing verbs. It doesn't seem bad, at first, but in the middle of smithing words it's easy to confuse.
Here's the simple part: lay is used with a direct object, lie is not. When you're doing it to something else, it's lay (I hear you snickering back there), when you're doing it to yourself (cut that out), it's lie.
I lie down. Bob lies on the sofa.
I lay the teddy bear down. Bob lays the book on the table.
Simple, see? Here's the part that gets folks: the past tense of lie is lay. The past tense of lay is laid.
I lay down last night. Bob lay on the sofa yesterday.
I laid the teddy bear down. Bob laid the book on the table.
Not content with just one transformation in the past tense, lie becomes lain as a past participle. Being a less restless word, lay is still laid in the past participle.
I have lain in bed for hours. Bob has lain on the couch all day.
I have laid the teddy bear down. Bob has laid the book on the table.
When describing things with a past tense narration, therefore, "the photo showed a man laying on his side," not "the photo showed a man lying on his side." The photo only shows a man lying down in the present tense.
I don't care what Microsoft Word says. The books are laying on their sides, not lying on their sides. Not unless they're caught in the act of laying themselves down, present tense.
So: Lie, lay, lain. Lay, laid, laid.
Don't blame me, I didn't invent the language.
Thanks are due to the Grammar Girl for clarifying this for me.