Here's what Moore himself said to Alan Wilkinson in 1980:
"At one time I was playing with a couple of pebbles that I’d picked up, because behind my far field is a gravel pit and there are thousands of shapes and forms and one only has to go out there and I can find twenty new little ideas if I wish, immediately. Anyhow, I was playing with two pebbles which I found like that and somehow or other they got locked together and I couldn’t get them undone and I wondered how they got into position and it was like a clenched fist being tightly … Anyhow, eventually I did get it to [separate]; by turning and lifting, one piece came off the other. This gave one the idea of making two forms which would do that and later I called it ‘Locking Piece’ because they lock together."
I like the simple, unpretentious language, that of a child almost. So much more direct than the art-speak used by many graduates of university art departments.