There are a myriad of answers to this seemingly simple question. Grading probably comes to mind for many faculty, but there is certainly more to the challenge of teaching than that. Some teachers blame students they deem ill-prepared, unmotivated, ungrateful, uncooperative, and entitled. Others might say that teaching takes time away from research, which is weighted more heavily toward tenure and promotion. Some might point to entrenched prejudice and systemic inequities in access to educational opportunities. Others yet might blame obsolescent teaching practices that have failed to keep up with rapid technological and societal changes. I think these causes miss the fundamental reason why teaching is hard: we simply don’t know the exact circumstances in which learning takes place. What precise conditions must arise to alter the brain’s functioning so that knowledge takes hold? No one knows. If we did know, we could create ideal learning environments for all students.