Empty Can
noun
A sky can is a can that has nothing but sky in it. It's an empty can.
The reading for 空き isn't one you've seen before. Remember how 開く means "to open"? Well, this can is also open, because nothing is filling it. The readings are the same between the two, as well. 開く and 空く, though in this case it becomes the stem form: 空き.
空き缶はここに入れてください。
Please put empty cans here.
なんでこんなとこに空き缶があるの？
Why are there empty cans here?
空き缶はリサイクルできるはずだよ。
Empty cans should be recyclable.