"Everything is relative."
Obviously a foolish statement to make, considering that it is absolute. However, I'll try to offer a (less-than-perfect) proof that absolutism is just as impossible to validate: (Feel free to challenge me on any of this in the comments ^.^ )
Assume the opposite - something is absolute. However, how can we know that something is absolute? By what reference can we know or declare it to be absolute? There must be something to compare something which is absolute to something which is not absolute (which is done even simply by calling it absolute) and that something makes it relative.
Any statement about something is inherently relevant, as an adjective only applies as a comparison, and a comparison is relative. For example, peanut butter is sticky and adhesive, but not compared to glue. However, this may not be valid (I see a shortcoming in my own logic) as it could be said that something which is absolute is not on any sort of scale of absolute-or-relativeness.
Here's another example – consider physical relativity: could there be a point in the universe somewhere that is absolute, or static? That would mean that while everything else moves, it does not. As such, there is nothing else to compare it to for stillness – furthermore, such a comparison would be to make the point relative again – ie., "This point is still - we know it because this point is still as well."
Just as "everything is relative" includes an absolute statement, "something is absolute" contains relativity as part of its definition, for that something is being compared.