Sometimes words in English are countable (you can add an -s) or not depending on their meaning.  A very common example is birds that you eat, such as chicken and turkey.  When you are talking about the whole animal (alive or dead), chicken and turkey are countable.

The farmer owned many chickens.

Since they could not find a large turkey, they bought two smaller turkeys for Thanksgiving dinner.

However, when you are talking about part of the animal, or the meat itself, chicken and turkey are uncountable.

I bought 1kg. of chicken at the store.

I ate so much turkey on Thanksgiving I felt sick.

Another word which can be countable or uncountable depending on its meaning is paper.  If paper has writing on it (handwritten or printed), it is countable.

Please pass your homework papers to the front of the class.

She left the important papers on the bus.

If paper is blank (no writing), then it is uncountable!

The printer was out of paper.

Students should never come to class without paper.

If you want to count blank paper you can use the counting marker “sheet” or “piece.”

I need two sheets of paper.

Can I have a piece of paper to write on?

Some other words which are sometimes countable and sometimes not are experience(s), fruit(s), and trouble(s).

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