Schematic of an experiment for observing counterflow between the normal and superfluid components of helium. (Left) At the closed end of the channel, a resistor (A) dissipates heat into the liquid helium. Heat is carried by the normal fluid with velocity VN toward the helium bath (B) at the other end of the channel. The superfluid flows with velocity VS toward the heater. If the heat flux is large enough, the superfluid becomes turbulent and forms a tangle of quantized vortex lines (C). Sharp tungsten needles (D) near the heater produce metastable helium molecules, which are transported by the normal fluid and detected by the probe laser (E) further down the channel. (Right) Fluorescent images of He2 molecules at different times show that an initial line of molecules remains a line, proving that the normal fluid has a flat velocity profile (F) consistent with turbulent flow, rather than a laminar parabolic profile (G).