If I understand this one correctly, one could very loosely "translate" it as follows:
"Barney, have you seen Fred?" -- asked Wilma Flintstone.
"He said he was taking the children to the lake, to throw pebbles into the water." -- answered Barney.
Wilma screamed and fainted.
(The word "галька" = "pebbles" , but "Pebbles" was also the baby daughter of the cartoon character Fred Flintstone. And in Russian, "Галька" is a nickname for the feminine name "Галина".)
P.S. A similar (but quite disgusting!) joke in English:
"Let's go back to my house and blow bubbles," said Michael Jackson to the little boy.
Explanation --> "blow bubbles" normally means "пускать мыльные пузыри", but in the context of Michael Jackson, one could interpret it to mean "делать минет моему шимпанзе Баблзу". <--