How to make the cat know where to poo

Question
I have two cats in my house, the bigger one and small one, the bigger knows where he should poo, and keep tidy, but the smaller one, don't know, always poo on the floor all the time,coz i think that normally the cat should know by their instinct when they see other cat do the same thing
what should i do and why?

Answer
You have to teach them. And keep the box very clean all the time. The other cat could have a problem if they don't use it after being taught. So you may want to talk to a vet. One vet once told me lock the cat in the bathroom till he or she learns to use it, yet that's hard and unfair. You should have a litter box on each floor of your house and always keep them clean. I recommend asking someone else how you teach them and also talking to a vet. Good luck and sorry