I used to do this. I admit sometimes I still do this, not that often, just sometimes when I'm babysitting and I want to keep the kids busy so I can go watch TV and eat all the food in their fridge.
There's a problem with this game, however. Often an insightful and inquisitive little fellow will ask you, "how do you play?" I don't take credit for the answer to this question, it's something that's always been customary where I grew up as a part of 52 card pickup, and it does enhance the game somewhat.
Kids like it, too. So here are the basic rules...
You hold up the entire deck of cards, with the front (the side with the numbers) facing the kid or kids, and you say, "if you see a black card, you say water. If you see a red card, you say fire." Kids love this kind of thing, it reminds them of Pokemon.
The fun part comes when they see a red card and declare, "Fire," in all their childish pride. Here's where you can say "Fire? Okay" and fold the cards back with your fingers in a way that flicks them in every direction, desirably in the direction of the child or children playing the game. You don't really have to say that, I think it's just a way to clarify the pun to younger generations of children.
That's the extended version of 52 card pickup, as I've always seen it played... maybe it's a regional variation, but I prefer the element of surprise and the way it anticipates any inquiry into the way of playing the game. Plus it has the nice effect of building up a child's enthusiasm and then ending up being a chore for them.