Complex sentence: It is often called "Einstein's Puzzle" or "Einstein's Riddle" because it is said to have been invented by Albert Einstein as a boy; it is sometimes claimed that only 2% of the population can solve it.
Split into simpler sentences: It is often called "Einstein's Puzzle" or "Einstein's Riddle" because it is said to have been invented by Albert Einstein as a boy. Some claim that only 2% of the population can solve it.