35 Words That Do Not Follow Plural Rules

Today I'm going to discuss a few words that don't form the plural by adding the letter 's'. These words come mainly from foreign 'imports', although a lot are very old. Some people are confused by these words and use the plural as a singular.
So here we go.

 Singular: Bacterium        Plural: Bacteria
 Singular: Phenomenon        Plural: Phenomena
 Singular: Medium            Plural: Media
 Singular: Datum            Plural: Data
 Singular: Criterion            Plural: Criteria
 Singular: Cactus            Plural: Cacti
 Singular: Fungus            Plural: Fungi
 Singular: Stadium            Plural: Stadia
 Singular: Nucleus            Plural: Nuclei
 Singular: Syllabus            Plural: Syllabi
 Singular: Focus            Plural: Foci
 Singular: Thesis            Plural: Theses
 Singular: Crisis            Plural: Crises
 Singular: Index            Plural: Indices
 Singular: Appendix        Plural: Appendices

It is becoming more acceptable to hear 'stadiums', 'syllabuses' and 'indexes', although they grate on me, personally.

Now for some that don't change for the plural.

 sheep
 deer
 fish (although the word 'fishes' can be used if referring to a number of different types of the creatures. e.g. There was a great variety of fishes swimming around on the reef.)
 aircraft
 moose
 offspring
 species
 salmon
 trout

Now what about those that are completely different in the plural? Here we have the following:

 Singular: Child            Plural: Children
 Singular: Man            Plural: Men
 Singular: Woman            Plural: Women
 Singular: Mouse            Plural: Mice
 Singular: Goose            Plural: Geese (N.B. The plural of 'mongoose' is not 'mongeese', but 'mongooses'.)
 Singular: Die             Plural: Dice
 Singular: Foot             Plural: Feet
 Singular: Louse             Plural: Lice
 Singular: Ox             Plural: Oxen
 Singular: Person             Plural: People
 Singular: Tooth             Plural: Teeth