Randy Zeitman in his answer says:
It's completely absurd to say we cannot be 100% certain.
He and others here have danced around a fact that needs to be made explicit. Certainty is a psychological state, and truth is generally assumed to be a logical state. Thus, we can be certain even when we are wrong, and uncertain even when we are right in matters of truth.
A simple example of that is a crackpot who pushes pseudoscience seemingly without regard to moderation or reason. Some have proposed tounge-in-cheek that there are essential characteristics or properties of such arguments and people that can be measured. Another example of misplaced certainty are the historical examples of entire groups of well-educated people who turn out to have been wrong in retrospect. Miasma theory was prominent when Ignaz Semmelweis built up a scientific argument that hand-washing before delivering babies saved lives. The reaction against his views were not only certain, they were outright hostile, and later in his life, his colleagues collaborated with his wife to have him committed to an asylum.
Now, as to the question of how we can be relatively certain we cannot be certain, this is a consensus position of epistemologists who almost overwhelmingly adopt fallibilism (IEP) as a position in regards to the certainty of epistemological methods. The short version is that when radical skepticism reared its head thousands of years ago, philosophers have been chipping away at the problem of eking out certainty, mainly by advocating deductive methods and showing the pragmatic nature of induction and abduction, to make claims that some certainty is possible while accepting that doubt is often justified. Today, such a position might be called contemporary skepticism on account that a lot has changed and been learned since the Pyrrhonism. Naturalistic epistemologies of scientific practice also align closely with this view that anything, with some prima facie justification, can and should be doubted. Most scientists agree that belief should be open to justification, which is a far cry from the certainty of a priori reasoning and introspection advocated by Rene Descartes.
If you want an even simpler response, most people are certain that there are reasons to be uncertain. Why? There is a set of very persuasive logical and psychological experiences that undermine certainty: illusion, delusion, confabulation, deception, self-deception, paradox, logical incoherence, lies, paltering, mistakes, errors, fallacy, psychological defenses, rationalization, cognitive distortion, and cognitive bias for starters. Anyone with a modicum of experience in life knows that these experiences are the norm, not the exception. Serious scientists, psychologists, and philosophers spend their lives trying learn about and spot these experiences to gain some certainty about what is going on in the world around them.