Skepticism is nothing new. Since ancient Greece, gadflies like Socrates and Pyrrho have questioned authority and sloppy assumptions held by the masses. These skeptics, however, did not get too far when it came to attaining positive knowledge about the world around us. This is because their skepticism merely consisted of tearing down knowledge and not constructing it. Modern skepticism differs from this ancient skepticism because it is not satisfied with merely questioning beliefs. It also wants to find out the how the world actually works and which methods are best for exploring it. To succeed at this task, the skeptics of our age have embraced, as Michael Shermer put it, "the most powerful tool ever devised for understanding how the world works." This tool is science.
What impresses skeptics about science is its long track record of successful prediction and potent application in fields like computing, medicine, and engineering. In the short time since the Scientific Revolution, science has explained the origin of species and shown us that our Milky Way galaxy is one among billions and billions. It has also been applied to eradicate diseases and to put a man on the moon. The skeptic is equally as impressed by science's methodology. Unlike traditional ways of looking at the world, science is built on probability (x is probably true or false) rather than certainty (x is definitely true or false). This means that all knowledge in science is fallible and can be revised if new information becomes available. This track record and openness to new ideas is why skeptics are confidant that science is the way to go.
Once we accept this, we can weaponize science by turning it into skepticism. This is done by taking its empirical methodology and collected knowledge and putting them in a toolkit with the best (those that are the most effective at judging knowledge and are in line with science) ideas from probability theory, philosophical logic, history and philosophy of science, and the study of folklore. The result is a baloney detection kit which is capable of exploring the history, social function, and truth value of claims ranging from physical theories about the universe to those made by used car salesmen to those pertaining to the paranormal. Like science itself, skepticism is committed to probability. When we examine the big bang theory and evolution, for example, we have to conclude that these theories are on the “almost certainly true” part of the truth spectrum because all of the science points one way. Others ideas, such as dowsing rods and astrology, fail testing so badly and go against our best scientific theories so dramatically that we must put them on the “almost certainly wrong” part of the same truth spectrum.
While it is certainly true that all skeptical knowledge is on a probability scale, this should not be mistaken for "every claim accepted by skeptics is not accepted with certainty, so I can believe what I want." There is so much evidence and corroboration for the ideas on the "very probably true" part of the spectrum (periodic table, evolution, and thermodynamics) that it would be perverse not to accept them. Likewise evidence points very strongly against ideas on the "very probably false" part of the spectrum (faeries, astrology, tarot). This tendency of skeptics to judge cherished beliefs based on evidence has led us to being labeled "rude", "cynical", or even"close-minded." While it may not be polite, these accusers need to be reminded what it was like when incantations, ritual, and witchcraft guided mankind's decisions about our health and the world we live in. Life was, as Thomas Hobbes put it, "poor, nasty, brutish and short."
If this approach is too confrontational for you, then you can remind them instead of the harm that being uncritical causes. Every year, thousands of dollars are thrown away calling psychics and reading horoscopes. Think of all of the people this money could have fed or how many people it could have put through college. Even worse, some people that have died using faith-healing rather than science-based medicine (no, I am not kidding). If only these people would have been more skeptical, lives could have been saved. If this view of the world sounds interesting to you, then I highly recommend reading through Carl Sagan's The Demon-Haunted World. This book is beautifully written and is full of lots of great information. You can also check out Brian Dunning's Here Be Dragons. This 40'ish minute video beautifully fleshes out a lot of the points made in this article. You can also check back here for more specifics or check out my facebook page for daily updates about science and skepticism.