One of the first texts I read that got me interested in philosophy was the Theaetetus by Plato and it posed this very question. It is a question which seems so simple on the face of it and yet the most obvious explanations quickly become mired in paradoxes.

Even though I was quiet young when I first read the text and have heard countless explanations since; I am still perplexed and as ever intrigued.

I'd really love to hear others views on this.