Once I watched a video on which a dozen or so children from a religious group are taken to their local natural history museum. The teacher/pastor tells the children to be skeptical of the exhibits and explanations for he has discovered that more than an institution, the museum is a temple based on faith. So he asks the children to be on the look out for unjustified claims, and ask 'How do you know?' each time any of these come up.
Sure enough when the museum guide is explaining one of the exhibits the children chirp 'How do you know?', 'How do you know?' to his dismay. (Wish I could find the video once more to confirm facts)
This sets an interesting quandary. I'm sure the guide knew his stuff, but, caught of guard and with short allotted time, he couldn't come up with a proper response on the spot. What would that response be? Certainly not rebuild all the theory from the ground up. Took me some years but I finally came up with something that could work.
"What we currently know is based on a body of knowledge that has been accumulated through decades, even centuries, of investigation by thousands of persons. The method by which we have accumulated this knowledge is called scientific, and relies broadly speaking in someone putting forward a possible explanation, called hypothesis, for the observations we have in our hands, and then having himself and others test it as rigorously as possible. This is the key part: we are not wedded to our hypotheses, in the sense that we are ready to let them go if these fail testing, which we call experimentation, or if a better one comes along that explains more or explains as much in a more simple manner.
"What we see here in the museum exhibits and in the words the museum staff and I give you are the best explanations we've got at the moment for natural history. Sometimes, we can't experiment directly to try to disprove our hypotheses, as in the case of what we think as long dead species. For these, we rely on evidence uncovered by digs. Very much as in detective stories, we build our case and, if it holds water, we accept the hypothesis as probably true. Once again, we are glad to revise our understanding if something turns up that contradicts what we already have.
"The beauty is that for each field of science, the knowledge, is internally consistent, meaning that it fits together, as a toy structure, and meshes well along with each of the other. Sometimes our hypotheses are so built, that they allow us to advance predictions. When these come true, by further observation, they add to our confidence. That is how we know."
He could even push for a bit more if he feels the audience as receptive. He could round of with:
"Oh, and science is Bible sanctioned. Remember Gideon from the Book of Judges? He wouldn't take things at face value, and tested, experimented, to be real sure about how things were."
