I have two boys, separated by 10 years. My older boy is a joy, well-behaved and smart as a whip. My two-year-old is a barrel of fun, too.
The best things I have experienced as a dad have revolved around seeing my children figure something out. When Sam, my elder son, figured out geometric congruence on his own recently, I was quite proud. When he was dissed on an online combat game by a bunch of kids in their late teens, he set booby traps for them and sniped some of the others. They have taken to asking him his opinion on how to do things.
The emergence of language with Sam was great fun. Charlie is just at the beginning of this right now. We have been teaching him animal sounds. He is quick to make the sound of any animal he sees, and will name an animal if he hears the sound. The acquisition of this has been very recent, and was pretty abrupt. Surely, he must have been piecing thing together, but its emergence as behavior has just in the past week or so, and it is very consistent.
We were playing, and he knocked over an end table that I have that has a built-in lamp. He thought this was great fun, and climbed on it. I wanted to right it, so I said, in a somewhat stern voice, "Move. Move!". To which he grinned widely and responded with "Cow!"
Reasoning comes more slowly, but I always push it, asking questions that I know that they cannot answer, mainly to get them in the habit of asking question, and to let them realize that answers exist. Years ago, I was playing with Sam with a helium balloon. I asked him what he thought made it 'fly'. He said, "I dunno, Daddy. What?" I expected the conversation to go back and forth. He was far too young to understand buoyancy, but I still think talking back and forth is great exercise for a child's mind. Even as an adult, I often hear explanations that I don't understand, but that I still keep deep in my head, and when I figure out enough, suddenly it makes sense.
I said, "Well, there is something inside the balloon that makes it go up into the air. What do you think is inside?" Sam thought for maybe two seconds, and said "Birds."