Every computer has a power button that turns it on and off. When you press the power button, the computer wakes up and shows you a login or home screen. When you're done, you should shut down properly using the shutdown menu, not just by pulling the plug.
Have children actually press the power button and watch the computer start up. Then practice shutting down properly using the shutdown option in the menu.
A computer is like a house that needs to be opened before you can use it. The power button is the front door — pressing it starts the process of waking everything up inside. When you press it, the computer runs through a startup sequence: it checks its own hardware, loads its operating system (the main software that runs everything), and then shows you a screen where you can begin using it.
The startup process takes a little time — usually from a few seconds to a minute or two. During that time, you might see a logo, a spinning circle, or a progress bar. These are all signs that the computer is doing its setup work automatically, just like how a car engine takes a moment to start before you can drive. You don't need to press the power button again or do anything else — just wait.
Once the computer is ready, it often shows a login screen where you type a username and password. This is how the computer knows who you are and loads your personal settings, files, and preferences. Think of it like clocking in at school or signing into a class — it connects "this computer" to "you specifically." If you're the only person using the computer, you might skip right past this to the home screen.
When you're done using the computer, it's important to shut down properly through the menu — not by pressing the power button or pulling out the plug. The shutdown process gives the computer a chance to save any open work, finish what it's doing, and store its state safely before turning off. Cutting the power suddenly is like slamming a book shut in the middle of writing a page — some of what you were doing might not get saved, and in some cases the computer can have trouble starting up correctly next time.
From your study of computer parts, you know that the monitor (screen) is separate from the computer tower or laptop body. Each has its own power button. Turning the monitor off just makes the screen dark — the computer is still running and you'd come back to see everything where you left it. Turning the computer itself off is the step that actually ends the session. When in doubt, always use the shutdown option in the menu rather than any power button.