Heli-dum, Part 2

I work in a craft shop. We sell balloons that we can fill with helium for an extra charge. I’m serving on the tills when two women come to my register. One is buying a pack of balloons. 

Me: “Hi there. Do you need the balloons filled with helium today?”

Customer: “No, I don’t see the point. If I just blow them up myself they’ll have air in them, so they’ll be light and float anyway.”

The lady’s friend and I then spent five minutes explaining why this wouldn’t work. Not sure we got through to her, as she didn’t buy the helium.

Related:
Heli-dum