There are many English words that you can add an “s” to make it plural. But far fewer words become singular when you add an “s”. What are two examples of these much rarer words?
You are the bus driver. At your first stop, you pick up 29 people. On your second stop, 18 of those 29 people get off, and at the same time 10 new passengers arrive. At your next stop, 3 of those 10 passengers get off, and 13 new passengers come on. On your fourth stop 4 of the remaining 10 passengers get off, 6 of those new 13 passengers get off as well, then 17 new passengers get on. What is the color of the bus driver’s eyes?
Smoke. It rises then slowly dissipates in the air. Smoke signals send messages and incense is used in religious ceremonies. Anyone who’s sat at a bonfire knows smoke in your eyes can make you cry when the wind changes direction. Smoked meats are a tasty treat, but if you see heavy smoke (i.e. from a fire) then run. Or stop drop and roll if the fire already caught you.