Talking?

We think that Katie is starting to talk. We certainly feel that she understands what we are saying, and she repeats the sounds we make.
When we ask her if she wants to get down from her high chair, she says "Da!" and she has been saying "Da da" (for Daddy) for quite a while.
When we are tickling her and playing with her, we trap her in our legs or arms. We tell her she is stuck, and she responds, "tuck tuck".
The other day I told her "no" for something or another, and she shook her head back and forth.
She has also really caught on to the concept of being chased. She will squeal and get all excited if we start chasing her. She will also want us to chase her, so she will run away when she thinks we are coming towards her. But, she always looks back to make sure we are really following her. She will even wait until we get closer, so she doesn't have an unfair advantage.