Sommer et al.  use radio-frequency (rf) spectroscopy to determine the binding energy of paired lithium atoms in a cold gas. This cartoon shows a gas with two species of fermions, denoted by red and blue, which are analogous to the spin-up and spin-down electrons in a metal. The presence of an optical lattice potential (blue curve) tunes the dimensionality and forces the gas into stacks of quasi-two-dimensional layers. The interaction between the two types of fermions is tuned using a Feshbach resonance. Absorption of the rf photon (red wavy line) converts a fermion in the red state to a different internal state shown as green, and measures the pair binding energy.