Just to make it for the 5%:
Mano -> Feminine
Imagen -> Feminine
Especie, planicie, parte, constante... -> Feminine
Azúcar -> Both feminine and masculine
Abad -> Masculine
Ion, camión -> Masculine
Pez, rapaz, cáliz, arroz -> Masculine
Problema, teorema, idioma... -> Masculine
Btw: Most plurals end with an "s", which are not to be confused with words whose singular ends in "s". These are normally made with a verb and a noun in plural; since we don't have neuter gender, we use the masculine and the word becomes invariable in either singular or plural:
· el/los rompecabezas
· el/los portafolios
· el/los sacapuntas
· el/los parteaguas
· el/los cascanueces ...