According to Lacan, in order for an object to count as an object a, two properties are necessary. First, the object must be a separable organ. Second, it must have a relation to lack.
Here is a story from Lacan about a sardine can.
“It is a true story. I was in my early twenties or thereabouts — and at that time, of course, being a young intellectual, I wanted desperately to get away, see something different, throw myself into something practical, something physical, in the country, say, or at the sea. One day, I was on a small boat, with a few people from a family of fishermen in a small port. At that time, Britanny was not industrialized as it is now. There were no trawlers. The fishermen went out in his frail craft at his own risk. It was this risk, this danger, that I loved to share. But it wasn’t all danger and excitement — there were also fine days. One day, then, as we were waiting for the moment to pull in the nets, an individual known as Petit-Jean, that’s what we called him — like all his family, he died very young from tuberculosis, which at that time was a constant threat to the whole of that social class – this Petit-Jean pointed out to me something floating on the surface of the waves. It was a small can, a sardine can. It floated there in the sun, a witness to the canning industry, which we, in fact, were supposed to supply. It glittered in the sun. And Petit-Jean said to me — _You see that can? Do you see it? Well, it doesn’t see you!
He found this incident highly amusing — I less so. I thought about it. Why did I find it less amusing than he? It’s an interesting question.
To begin with, if what Petit-Jean said to me, namely, that the can did not see me, had any meaning, it was because in a sense, it was looking at me, all the same. It was looking at me at the level of the point of light, the point at which everything that looks at me is situated — and I am not speaking metaphorically.
The point of this little story, as it had occurred to my partner, the fact that he found it so amusing and I less so, derives from the fact that, if I am told a story like that one, it is because I, at that moment — as I appeared to those fellows who were earning their livings with great difficulty, in the struggle with what for them was a pitiless nature — looked like nothing on earth. In short, I was out of place in the picture. And it was because I felt this that I was not terribly amused at hearing myself addressed in his humorous, ironical way.”
From Lacan’s book. – The Four Fundamental Concepts of Psycho-Analysis.