The river
The fox, chicken and grain problem was a logic puzzle in which one had to get a fox, a chicken, and a bag of grain safely across a river, one at a time, using a small boat. The objective was to not allow the chicken to eat the grain or the fox to eat the chicken, if left together.
By the 24th century, Hologram Janeway used this puzzle on the holodeck to help teach the crew of the USS Protostar how to work together. While several options were explored by the Protostar crew, when it was suggested by Zero that "if we take one across, there's no rule to say we couldn't take another back," they were informed by Janeway that they were "getting warm." There was also an added degree of difficulty in using the holodeck, as the animals had to be kept under control, and the chicken had the capability to attack the participants.
While stuck in a temporal anomaly, Rok-Tahk watched a recording of the crew attempting to solve the problem and commented that they should've listened to Zero. (PRO: "Time Amok")
