The Oscar-winner had stashed a bag of the psychedelic fungi in his fridge and was shocked when his pet Lewis pulled the door open and munched on the mushrooms.
So Cage decided to join in - and he and Lewis then spent hours staring at each other in his bedroom.
He tells chat show host David Letterman, "I had a bag of mushrooms in my refrigerator. My cat used to sneak into the refrigerator and eat them. The cat ate them voraciously, so I thought what the heck, I better do it with him.
"I remember lying on my bed for hours and Lewis was on the desk across my bed and we just stared at each other for hours - not moving, just staring at each other, and I had no doubt that he was my brother.
"But having said that I don't do that anymore. And you know what, later in life when I was completely not doing any of that, I know he said 'Hi' to me."