At what level does annihilation occur? For example, if I've got an up quark and an anti-up quark, they can annihilate. If I've got a proton (uud) and anti-proton (anti up, anti up, anti down), they can annihilate. What if I mix a neutron (down, down, up) with an anti-proton (anti up, anti up, anti down)? Can we get the annihilation of the neutron's down quark(s) with the anti-proton's anti-down quark, or of the neutron's up quark with the anti-proton's anti-up quark(s)? What if we have an anti-H atom (anti-proton and positron, I guess) - obviously this will annihilate with an H atom (proton and electron), but will the anti-proton annihilate individually with a proton, or the positron annihilate individually with an electron, to leave behind just a positron or an anti-proton respectively? Will an anti-H atom annihilate with an H2 molecule, given that the H2 electrons are taken up in a bonding pair? What will be left afterwards then?