The optimal video length is 6 minutes or shorter -- students watched most of the way through these short videos. In fact, the average engagement time of any video maxes out at 6 minutes, regardless of its length. And engagement times decrease as videos lengthen: For instance, on average students spent around 3 minutes on videos that are longer than 12 minutes, which means that they engaged with less than a quarter of the content.

However, nobody would assume that in order to know that wood one has to know exactly every tree, one by one, its shape, age and location. Every plant. Every leave of every plant. Every animal and where every animal is and what every animal is doing at any instant. Every stone. Every particle.