You can make a "regular" noun plural by adding "s" to the end of it (dog/dogs, cat/cats, etc.). Many nouns, however, are "irregular" in their plural form: words like "tooth/teeth," "calf/calves," "man/men," "mouse/mice", "moose/moose" etc.
Irregular Plurals Fun Deck offers a great way for children to learn these more difficult plurals. It has 26 illustrated singular and irregular plural word pairs, plus bonus cards and game ideas. Cards are stored in a sturdy storage tin.