What is hidden in 3D image?

A lady went to the bar on a cruise ship, and ordered a Scotch, with two drops of water. The bartender gave her the drink, and she said, "I'm on this cruise to celebrate my 80th birthday, and it's today."
The bartender said, "Well, since it's your birthday, this one's on me."
As the lady finished her drink, a woman, to her right, said, "I'd like to buy you a drink, too." The lady said, "Thank you, how sweet of you. OK, then, Bartender, I want another Scotch, with two drops of water."
"Coming up," said the bartender.
As she finished that drink, a man, to her left, said, "I'd like to buy you a drink too." The lady said, "Thank you very much, my dear. Bartender, I'll have another Scotch, with two drops of water."
"Coming right up," the bartender said.
As he gave her the drink, this time, he said, "Ma'am, I'm dying of curiosity. Why the Scotch with only two drops of water?"
The old woman giggled, and replied, "Sonny, when you're my age, you've learned how to hold your liquor. Water, however, is a whole other issue."