Riddle: What type of ship only has two people in it?
Answer: A relationship.
Riddle: What number can you subtract half from to obtain a result of zero?
Answer: The number 8.  It's made up of two zeros, one on top of the other.
Riddle: What two whole, positive numbers have a one-digit answer when multiplied and a two-digit answer when added?
Answer: 1 and 9.
Riddle: What two whole, positive numbers have the same answer when multiplied together as when one is divided by the other?
Answer: Any number and 1.
Riddle: What two whole, postive numbers have the same answer when multiplied together as when added together?
Answer: 2 and 2.
Riddle: What two-digit number equals two times the result of multiplying its digits?
Answer: 36 = 2 x 3 x 6.
Riddle: What three whole, positive numbers have the same answer when multiplied together as when added together?
Answer: 1, 2 and 3.  1 x 2 x 3 = 1 + 2 + 3 = 6.
Riddle: What two-digit numbers are each equal to their right-most digit squared?
Answer: 25 = 5² and 36 = 6².
Riddle: What is the highest number that can be written with three digits?
Answer: (9⁹)⁹ = 9³⁸⁷⁴²⁰⁴⁸⁹, which is a number with more than 369 million digits.
Riddle: What do you call a tree that has been TP'ed?
Answer: A toiletry. (Toilet-Tree)