I'm staying with my friends in Tulsa. One of their toilets has a weak flush. I opened up the tank and inspected the flapper. It appeared to be okay. Then I flushed the toilet and ran my finger along the bottom of the flapper. My finger had all this black gunk on it. Ewwwww. The flapper was disintegrating and dislodging black chunks of rubber. Another toilet in the house has the same type of flapper. I shut off the water supply to the one toilet and removed the flapper. I put the flapper in a plastic bag and took it with me to the hardware store.
It is amazing to me how many different types of flappers there are: big, small, water-saving, brand name, etc. I got two of the basic flappers that were the same size. I had to use the restroom at the hardware store to remove some of that black rubber gunk that got onto my hands.
I got back to their house and in just a few minutes had the flappers replaced. I even removed some film from the inside of the tank with a rag. Both toilets flushed properly. Yea! A successful plumbing job with only one trip to the hardware store. That almost never happens.