The girls have given you good advice, I personally tried not to use NO ever when DD was little. I found despite my best intentions I used it a lot more when DD got older! But it sets up a good habit for trying to find an alternative way to explain things, like Stop, that's hot, it will hurt. Simple, but effective.

I actually think the best thing to do is to distract her. She's so little, she's interested in what is in front of her and she wants to explore. Instead of worrying about her learning not to go near it, which as marcellus says requires impulse control that she doesn't have yet, just draw her attention somewhere else. Grab something else she really likes and wave it in front of her. I bet the heater, the drawer and the TV cabinet are less interesting then!

I would still say, stop, that's hot em, look what Mummy's got instead! So get the message out, but don't expect her to regulate her behaviour when she's not old enough to do that. You wouldn't expect her to go and answer the phone politely when it rings, neither can you expect her to manage her behaviour and impulses at the level of a four year old (and she'll still have trouble even at that age with some things!).