Another Awkward Professor Encounter

What is the appropriate way to tell your professor that you never had any intention of doing an assignment he/she had given and subsequently collected from the rest of the class a week ago?

I get this all the time, (well not ALL the time, I do SOME work)... But it happens quite a bit at Augsburg. Today, it happened again.

I breezed through my Spanish test, mostly because we go slower in Augsburg Spanish than we did in high school Spanish... (what does that tell you?), only to have the professor stop me as I was leaving.

"Mark, I can't seem to find (this five point) assignment from you."

And... I'm trapped. What I'd like to say is the truth...

"Oh, that's because I didn't do it."

But that would lead to this:

"Why?"

Which would force an answer that would probably end the conversation in a bad way.

"Well you see, I didn't feel like THAT MUCH work was worth the five points it would bring me, especially when it's a 1000-point class. You see that test I just filled out? You're giving me 100 points for that... well, you're giving me at least 94 points because I nailed it, and I finished that test about the same amount of time it would have taken me to do that five-point homework assignment. FIVE POINTS. It just makes more sense to not waste my time."

But we all know I can't say that. I can never say that, no matter how many professors ask. So instead, the conversation goes...

"Mark, I can't seem to find (this five point) assignment from you."

"Oh."

And I still look dumb, because there's nothing more to say.