Here's a place for us to bore eachother to death about grammar. I'll go first.
I. POSSESSIVES AND PLURALS
Most nouns are made plural by the addition of 's' or 'es'. computer -> computers house -> houses
Some nouns have irregular plurals. mouse -> mice fish -> fish
A noun is never made plural with an apostrophe.
The apostrope and 's' are used to show possession. Ron's (belonging to Ron) Ginny's (belonging to Ginny)
However, pronouns have their own possessive forms: his (belonging to him) hers (belonging to her) theirs (belonging to them) its (belonging to it; it's means 'it is')