Learning something new — and quickly — may depend on the lesson’s difficulty level, according to a new study.

Flipping the classroom, room temperature, and later school-day start times, are just a few of the countless interventions scientists have tested and some educators have implemented.

Now, scientists say they have cracked the code on the optimal level of difficulty to speed up learning. The team tested how the difficulty of training impacts the rate of learning in a broad class of learning algorithms, artificial neural networks, and computer models thought to simulate learning in humans and animals.