We form the plural of nouns that end with a consonant + “y” by changing the “y” to “i” and adding “-es:” For example, <em>city - cities, lady - ladies, activity - activities</em>. The only exception is when creating the plural of proper nouns ending in consonant + “y” such as <em>Kennedy, </em>whose plural is<em> Kennedys</em>.
On the other hand, when a noun ends with a vowel + “y”, we form its plural adding an “s” only. For example, <em>monkey - monkeys, boy - boys, valley - valleys.</em>
This is part of Tom's plan to entice the other boys into painting the fence for him. He plays on their emotions in order to convince them that painting the fence is a "fun" thing to do.