Veritasium has their response to the bullet/block I experiment I linked to and explained, and then explained some more. I absolutely love that they were overwhelmed with responses as people tried to figure it out.

One nit: momentum is not always, always, always, always conserved. It’s conserved when you define the system such that there is no net external force on it. The bullet-block system can be said to conserve momentum during the collision, because it happens quickly and the perturbing impulse is small. But over a longer time, the momentum clearly changes, as the block rises and then falls. One would have to incorporate the earth to have momentum be conserved, but that wouldn’t help solve the original problem. Defining the system so that one can come to a solution is a large step in the setup to any problem.