The answer to the riddle "From what number can you take half and leave nothing?" is the number 8, because when either the top or bottom half of the digit is removed, the remaining half forms the number zero, or nothing.
Another visual number riddle involving the number zero is "Why is the number 9 like a peacock?" The answer is because it is nothing without its tail. If you remove the tail from a number 9, the remaining circle is a zero. Another visual number riddle is "What is the difference between 100 and 1,000?" The answer is zero, because 100 has two zeros and 1,000 has three.