How many dollar bills in a ton?
Test your own metric intuition: Predict how many pennies, paperclips, and dollar bills are in a kilogram, and how many are in a ton, and put your answers in the comments. Then, if you have access to a balance, you can measure; otherwise, I'll share my students' results in a few days.
I like teaching this stuff. The kids walk away with skills they didn't have at the start of the week. It's so easy to tell who learned it and who didn't, and to get that sense of gratification of knowing that you actually taught them something.
(NB: They are the large Staples paperclips coated in colored plastic, if that makes a difference to anyone's estimating).