Notice where we put the "s" on the end of a word and where we don't:
- Thousands of people watched the show.
- Fifty thousand people watched the show.
- over a period of ten years
- over a ten-year period
- The fish weighed 5 kilos.
- It was a five kilo fish.
- The boy is three years old.
- He is a three-year-old boy.