Copy and paste
Engelbart's demonstration started out, like most projects, with a blank screen. To populate it with data, he performed a function that is so mundane to our eyes, it's hard to conceive how revolutionary it was at the time: He selected some text, copied it, and pasted it repeatedly.
To get an idea of how brilliant copy and paste seemed, think back to the first time you saw Microsoft Excel's Autofill: You type the word Tuesday into a cell, drag the cell down and Excel automatically advances the days of the week into each successive cell.
Your smartphone's autocomplete function takes things a step further, anticipating your intent before you even finish typing a word. And isn't that what augmenting human intellect is all about?