When one cloud of gas meets another, they normally pass right through each other. But now, physicists have created clouds of ultracold gases that bounce off each other like bowling balls, even though they are a million times thinner than air — the first time that such impenetrable gases have been observed. While this experiment involved clouds of lithium atoms, cooled to near absolute zero, the findings could also help explain the behavior of similar systems such as neutron stars, high-temperature superconductors, and quark-gluon plasma, the hot soup of elementary particles that formed immediately after the Big Bang.