It's a bit tricky because it ends in an "o", which is usually associated with feminine nouns. However, there are many exceptions to this rule in Spanish, and "mano" happens to be one of them.
Here are some reasons why "mano" is masculine:
* Historical reasons: The word "mano" comes from the Latin "manus," which was a masculine noun.
* Usage: In Spanish, the word "mano" is always used with masculine articles and adjectives, for example, "el mano" (the hand), "un mano" (a hand), "un mano fuerte" (a strong hand).
So, while the ending might suggest otherwise, "mano" is undeniably a masculine noun in Spanish.