the original derivation of E=mc2 (it's not difficult!)

How did Einstein derive his famous expression E=mc2? This 25-minute seminar reconstructs his reasoning, which - quite remarkably - can be told using concepts that are familiar to anyone who has ever taken introductory physics classes at school. By the end of this video, you will understand why boiling water is heavier than cold water.
Yes, this is the same explanation as given in the great 2-minute video by minutephysics at • Einstein's Proof of E=mc² . Only, in the present one, the pace of the arguments will be more digestable for many of us ;-).
Edit 1: Tibees has a recent video ( • Why haven't you read E... ) with a similar story line. Very complementary to this one, recommended!
Edit 2: AI-generated (Whisper) subtitles and translations added.
(recorded at a group seminar at Ghent University)