Around this time, a toddler begins to understand that "no" is what Mom says when she doesn't like what he's doing ("No! Don't stick your fingers in that outlet!") and "yes" is what she says when she's okay with a request or behavior ("Yes, you may have a cookie"), says Vicki Panaccione, Ph.D., a child psychologist with the Better Parenting Institute in Melbourne, FL. So, knowing this, what does a toddler do? Say yes to everything, of course!
To set him straight:
Read his body language. It's pretty obvious: If he blurts out "yes" but is scrunching up his face and refusing to eat his peas, he probably means "no." If he's saying "yes" and his eyes look excited, you can assume it's a go.
Give him other choices. For instance, instead of asking if he wants to go outside, ask whether he wants to go out or stay in.
Explain the difference. Once he answers, help him understand that he said "yes" to going outside and "no" to staying in. Do this consistently and he'll get the hang of "yes" and "no" quickly.