What is hidden in 3D image?

A musical director was having a lot of trouble with one drummer. He talked and talked and talked with the drummer, but his performance simply didn't improve.
Finally, before the whole orchestra, he said, "When a musician just can't handle his instrument and doesn't improve when given help, they take away the instrument, and give him two sticks, and make him a drummer."
A stage whisper was heard from the percussion section: "And if he can't handle even that, they take away one of his sticks and make him a conductor."