3 lessons
What is an operating system, really?
Why your computer needs a manager at all, and the one idea underneath everything an OS does.
System calls — how programs politely ask the kernel for help
The doorbell every program rings when it needs the OS to do something on its behalf, and why it can't just walk in.
How a computer wakes up, and how it reacts to surprises
What actually happens in the seconds after you press the power button, and how the OS handles events it didn't ask for.