LOOKING FOR ANTI-QUARKS INSIDE PROTONS. Consider a proton zipping through the lab. Only about 30% of its momentum is taken up by valence quarks (two "up" and one "down" quark), the nominal constituents of the proton. Gluons, which keep the quarks attached to each other, account for 50% of the momentum. And the rest, the other 20% of a proton's momentum, is take up by virtual particles which pop pairwise into and out of existence on short-term loan from the vacuum (sometimes referred to as the "Fermi sea"). Like a reclusive tiger photographed at a waterhole with a tripwire camera, one of these virtual particles can be viewed if it is struck at just the right moment by a probe particle from a high-energy beam. In this way, the anti-up and anti-down quark population of protons has been systematically studied in a scattering experiment at Fermilab (contact Paul Reimer of Los Alamos, preimer@lanl.gov, 505-667-0145). In general the prevalence of anti-u and anti-d quarks is about equal, except for cases where the struck quark has low momentum; then anti-d's outnumber anti-u's considerably. (Hawker et al., Physical Review Letters, 27 April.)