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For examplethe theory of the strong force, QCD, consists of point quarks heldtogether by gluons. At high enough energy, experiments can see what look like almost free quarks weakly coupled to gluons. However, there is -- of course -- some smallest scale that such experiments can probe, while the the theory consists of point quarks. When you look in a "small box" of space you actually see is a quark (say) plus a bunch of gluons and quark-antiquark pairs. What is actually observed,then, is a "quark" which is actually a combination a quark and a bunch of gluons and virtual quark-antiquark pairs, and a "gluon" which is actually as combination of gluons and quark-antiquark pairs.