See Martin Heidegger, The Question Concerning Technology (English ed, Garland, 1977):

Whatever stands by in the sense of standing-reserve no longer stands over against us as object. Yet an airliner that stands on the runway is surely an object. We can represent the machine so. But then it conceals itself as to what and how it is. Revealed, it stands on the taxi strip only as standing-reserve, inasmuch as it is ordered to ensure the possibility of transportation.

Thus,

What makes something a tool

is its being not autonomous: tools, unlike objects, that "stand by their own" need humans.