what is the easiest way to tell my dog he was adopted?

Make him a pumpkin cake that says “you’re adopted!” 

Have him/her read Yahoo! Answers (this question). 

Explain that even though you and he look identical, he has different parents and the similarities are a freak of nature.

Explain that even though your IQs are in the same range, that is also a freak of nature.  One of you, I'm not sure which one, posted this:  "what is the best time to begin your first transformation from a maggot in to a cow?   my sister has just started and i'm a bit older and i don't know when" 

Little kids aren't supposed to be on this site.

Never tell your fur babies they're adopted!

roflmao   if he is loved cared for and happy do you really think he or she cares? 

Usually you and the dog sit on the couch together and you bring up how nice it is to be together and the good time when you first met.  He will remember that happy moment and then you tell him that was when he was adopted.  It will suddenly occur to him what happened, but he will be happy about it and not disappointed.  A few hugs and everything will be OK.

when he is asleep