This is my husband's solution for how to determine when my disposable razor is used up. He proposed a fully scientific method of marking my next fresh razor with a start date, tracking how long it's good for, then writing an expiration date on each subsequent razor as it's put into use.
I told him that was a ludicrous idea.
"Why don't you post it and see what everyone else thinks."
And so I am.
I still prefer the method that prompted this discussion: "Ouch. Is this thing used up already?"