"Suicide By Squats", a brutal "How long can you last?" exercise.
Blog writeup: http://regex.info/blog/2017-09-23/2825
Set a timer. During the first minute, do two deep, proper-form squats. Do four during the second minute, six during the third, and so on, adding two for each minute. When you fail to be able to do the squats for that minute, you're done.
Head started throbbing again toward the end of Minute 21, so I laid off Minute 22. (Wouldn't have finished it anyway.)