As a former chemistry and physics teacher, one thing I found to make a huge difference in helping students think about science (or really any) concepts is asking the right questions. I have learned that it's best to stay away from "yes" and "no" questions and focus on questions that require a more detailed response.
"What do you think will happen when I...."
"What happened when we....."
"Why do you think that happened?"
Staying at home with my 2 year old- I'm working on creating little science moments (not major science experiments that require a lot of setup) that will help her to start thinking like a little scientist. Many of my postings, will explain how I created these little moments and give examples of questions I asked her. One thing to note is that my two year almost never answers the questions "correctly"...but that's not point. My goal is not to make sure she has a deep understanding of density, light refraction, etc. (though eventually, I hope she will learn these things as she grows up). My goal for now is to help her to start thinking in a scientific way- which, I hope, will help her in all areas of life.
These science moments can be created most anywhere and anytime with a little creativity; at the park, in the car, in the kitchen, on the stairs, playing with toys, etc.