I've read that mesons consist of a quark and an antiquark. So, here's my question. Why don't the quark and the anti-quark annihilate with each other (like they ususally do)? For example, the pi(0) meson consists of the up and the anti-up quark and the eta meson consists of the down and the anti-down quark and even the eta-prime meson which is made up of a strange and an anti-strange pair of quarks. So, what's the reason for the existence of these mesons? What causes a quark and an anti-quark to come into a bound state and form a meson?