Apparently Turing thought that it's impossible to fool a human into thinking that someone is thinking. We can be fooled into thinking that a straw inside a glass of water is bending, we can be fooled into thinking that a dream is reality, but if we think someone is thinking, we can't be wrong.

Next time I see the statue of the thinker from a distance and mistake it for a real person and form inside my head the notion that 'this person over there is thinking', that will be proof enough that rocks can think :D