They Didn't Teach Me This in School

The other day I was driving around town and had Little Guy with me.  It had been mostly quiet driving around because LG was chewing on his fruit snacks.  All of a sudden from the back I heard, "This says zero.  That means it didn't cost anything."  I glanced back to see what he was talking about, and realized he was holding up the fruit snack package and looking at it.  One of the numbers on there said 1.0 oz or something similar.  I said, "Oh, no, it costs money.  That just means how much the snack pack weighs."  He went silent for a moment and then said with authority, "No.  Then if it is a zero, that means it doesn't weigh anything."

I was impressed with his ability to deduce that from what I told him, but shocked at my own inability to explain a decimal point to him.  I tried to explain that it was just a number to tell us how much all the snacks weighed, but he kept coming back to the zero because he knew it meant a quantity of nothing. 

People, I have a masters degree and am semi-skilled with numbers.  I couldn't think of how to explain this to a four-year-old.  Decimal points.  Freakin' decimal points.  Something tells me I need to stop lecturing to undergraduates, and start prepping myself for five-year-old math questions.  Something also tells me this feat will be harder.