Johns dog was named Spot.
Spot wagged it's tail.
"Its going to be a good day." he said.
"Your a good dog."
John offered some dog biscuit's to Spot.
(i.e. it doesn't make sense to say The dog wagged it is tail).
John offered some dog biscuits to Spot. The word biscuits doesn't need an apostrophe because 1.) it
Here's another tricky part (darn those tricksters who made up the language rules!): occasionally, an apostrophe is used to form a plural of a family name that ends in es, like the Jones's and to form the plural of a group of years, like the 90's.