Neuron burner

A neuron burner is a problem that you need to think long and hard about. It's similar to "being in the zone" in terms of the effort of concentration, but "in the zone" does not describe the difficulty of the problem, which"neuron burner" does.

"The instructions for this microwave are a real neuron burner"
"This test was one neuron burner after another! Glad it's over."