There is a module built into Adapt to handle exactly this, it's called Notify and is documented here. Many of the Adapt plugins will be using this (the Hot Grid component Sam mentions in his reply for example).
You can also trigger it manually in any Adapt course if you want to see it in action - quite useful for being able to see all the different options available to you.
With an Adapt course running in your browser, open the developer tools (F12) then go to Console.
Type/copy-paste in the following then press Enter:
var Adapt = require('core/js/adapt');
This creates a reference to Adapt. Now that you've done that you can trigger Notify in the course by running commands in the Console such as:
Adapt.trigger('notify:popup', {title: 'this is the title', body: 'this is the body'});
Or, one that's sadly little-used but which I rather like:
Adapt.trigger('notify:push', {body: 'Ah, push it (get up on this)'});
See the documentation for more examples.