Encouraging a baby's language development is a lot like pulling taffy. Taffy becomes better formed when it is stretched and pulled. This is true of language, too!
You can accept your baby's idea, and then stretch the idea just a little. When you add a concept word, you are giving your baby a chance to see an idea used in a more advanced way. That encourages your baby to stretch ideas and think to the next step.
The important point is to stretch just a little, so that your baby still recognizes the original idea. You do this by adding just one idea or putting the child's thought into a slightly longer phrase. Here's an example:
Baby: "Ball!" Parent: "Yes. A
Babies usually mean more than they say. "Ball!" might mean "Big ball!" if the ball is bigger than usual. "Ball!" might also mean "Roll me the ball!" You can say, with your eyebrows up and your pitch rising, "Roll the ball?" Your child will usually let you know what "Ball!" means by adding gestures and facial expressions.
You are still responding to your baby's lead, but stretching the baby's idea adds to the concept. Notice how the concept "wet" gets used many times….