Introductory animation to blockchains

Imagine that the envelopes are transparent, so the wax seal (the "Proof of Work") serves to protect the integrity, but not the confidentiality, of enclosed data. Notice that the more deeply nested a transaction becomes, the more seals one would have to break and reseal in order to modify it undetected. Even worse, since the mining continues to nest on top of that transaction while you are mounting an attack, you have to race and overcome all of the mining which takes place over the duration of the attack!