This is The Bloody Bath Mat ($15/regular, $20/large), an ultra-thin 0.03-inch plastic bath mat sheet that changes color from white to blood red anywhere it gets wet. Some more info while I add a regular bath mat to my Amazon cart because I don't have one and my girlfriend thinks I "live like an animal" and "GET SOME F***ING TOILET PAPER ALREADY":
Try the epic Bloody Bath Mat™ sheet prank, a thin plastic mat that reacts with water to color change your wet footprints into blood red stains. Not your typical mat, these thin color changing prank sheets are less than 0.03in thick, giving your friends the shock of their life with this unique bathroom prank as they leave a trail of bloodstains out of the shower. This mat dries back to white after each use so you can scare again and again! Get this horror mat and be ready for Halloween or just random hilarious pranks!
Oh man, could you imagine stepping out of the shower onto this and not knowing it was a prank bath mat? You'd probably think you accidentally cut your penis off in the shower! "Sounds like you're speaking from experience, GW." Please don't play the 911 call if you have it.
Keep going for a video of the mat in action.
Thanks to Allyson S, who agrees are you even having fun if you didn't just convince somebody they hurt themselves?