What words plural form is the same as its original form?

There are several words whose plural form is the same as their original form. These are called zero plurals or singularia tantum. Here are a few examples:

* Sheep: One sheep, many sheep.

* Fish: One fish, many fish. (Although you can also use "fishes" to refer to different species)

* Series: One series, many series.

* Species: One species, many species.

* Means: One means, many means.

* Corps: One corps, many corps.

* Offspring: One offspring, many offspring.

* Head of cattle: One head of cattle, many head of cattle.

* Aircraft: One aircraft, many aircraft.

* Data: This one is debated, but "data" is often used as a singular noun.

It's important to remember that some words, while they can be used as zero plurals in certain contexts, can also have regular plural forms. For example, "fish" can be used as a singular noun ("I caught a fish") or as a plural noun ("I caught many fish").