Yes, "girls" is a plural noun. It refers to more than one girl, the singular form being "girl." In English, adding an "s" at the end of a noun typically indicates that it is plural.
Buffalos
Shelves
Mitochondria
cargoes or cargos
sheep, it doesn't change.
The plural of activity of activities.
Children or kids
Crises -- pronounced krahy- - seez
Successes is the plural form of the noun success.
oxen - eg. a team of oxen pulled the plough
phenomenons; for example, they were hit with many phenomenons. :) hope this helpss :D
The plural of calculus is generally "calculi." The word "calculuses" is also a plural, but very rare.