Given a set of five words, generate the word from the set that does not belong (i.e. is the least relevant) with the other words. Words are separated by commas.
One example: excavator, crane, pelican, hoist, upraise
Solution is here: pelican
Explanation: The common category here are words related to construction. A pelican is not related to construction because it is a type of bird.

Now, solve this: unhappy, down, sliding, lower, flat
Solution:
unhappy