This was Cassie responding last night to the Best New Thing in the World segment in which we met Jack Green, the British sprinter who wrote training reminders on his own arm to assure he remembered good running form.
I did a quick google and found a lot of silly running-themed temporary tattoos, but what I think Cassie is really talking about is something like this:
From a site called PaceTat.com, it goes on your arm (and comes off) with like any other novelty tattoo. The site says they offer different calibrations for different runners. The ratings look like this: 4:00 / 2:00 – 9:09 which means a 4-hour marathon, 2 hours to the half and a 9:09 minute first mile.









