Wear one for four days straight:
First day, you wear it the normal way, fresh out of the package.
Second day, you switch front and back (i.e. wear the Y-front in the back)
Third day, you turn it inside out but wear it the correct way around (Y front in the front)
Fourth day, you switch it around again, so you're wearing it inside out with the Y-front in the back.
Your choice how many stains you want to leave in that pair of tighty whities on the first two days that will obviously show up on the outside the last two days