Have you ever seen a baby crawling around learning things? The next time you're around a baby who's old enough to move (i.e., >4 months or so), sit down and have some playtime. It's quite a joy to watch: checking out new things, going back to old ones - shaking, pounding, tasting, smelling, touching, feeling, listening, using every sense to try to figure out how it works.
What's going on in their heads? We can't really know for sure, but we can guess. They're trying to figure out how the world works. There's a world around them, and they don't know what's going on -- so, characteristically, they launch into figuring it out. You might characterize this as building expectations for how things should behave. In other words, they're developing models for how the world works. Their expectations are based on predictions the model makes.
Babies are natural scientists. It's informal, sure, but the informal scientific method is there: poke at it, watch its response, and take it into account in your mental model of how it works.
It's a shame we adults have to relearn this -- a lot of us never actually do. Some lucky people never lose it, but after a couple of decades, the old mental shortcuts kick in, and we forget what it's like to actively learn. We forget how to engage life, how to approach new ideas skeptically, how to be unsure, and in fact be sure of just how unsure you are. (To be fair, the "adult" version of the scientific method is a lot more rigorous - just the other day, I measured G to be (5.3 +/- 1.2) x 10^-11 Nm^2kg^-2*; that sort of quantification is far, far beyond my daughter. But the basic idea of the scientific method is the same, and when you apply it to life in general, you can't always specifically quantify your uncertainty; you just have to get a feel of how off your best guess might be and go with it.)
Part of the reason for this forgetfulness is the mental shortcuts we figure out as we go ahead in life. When you're figuring out something new, like the fact that things fall when you drop them (wow!), you're very tentative. As you experience it more, your uncertainty decreases. And after a while, your uncertainty gets so low that it's not worth your time to think about it anymore: you just assume that when you drop something, it will automatically fall - end of story.
But that black-and-white thinking, while convenient, is harmful. As adults, we face a world full of uncertainty -- and if we've gotten sloppy by not engaging life, we have simply forgotten what it's like to be uncertain! Thus breed black-and-white thinking and other symptoms of intellectual sloth.
So, moral of the day? To be skeptical, be more like babies -- get a sense of your uncertainty, and realize that the things you "know", you really just have very high confidence in because you have a lot of (experiential) evidence** for them.
* Which is cool in and of itself; how many people can say they've measured Newton's gravitational constant to within 20%?
** Mind, it's often good to go back and requestion these things too once you have the adult version of the scientific method, but all too many people don't even have the baby version. Once you get the adult version, you'll sort of automatically question them anyway.