Feet or foot are similar words .
GIVE REASONS also
Answers
The same reason it's geese for goose, lice for louse, mice for mouse, etc...
It has something to do with an old English rule called I-mutation. It is the raising and fronting of a root vowel in anticipation of "i" or "y" sound in a suffix.
For example, think of the difference between the 'do' in "How did you do it" and "What are you doing". When that -o- shifts up to an -i-, that's i-mutation. Simple! I-mutation is essentially because we are lazy. We prefer taking the shortest distance between two points.
I found this on the internet:
"The plural of man in ancient West Germanic, the ancestor of Old English, used to be a word something like *manniz. The speakers "cheated" on the first vowel in the word to be in position for the second vowel.
So after hundreds of years of this, the plural came out as *menniz, or something similar, when people said it. Eventually, the shifted vowel itself comes to stand for the plural, and the syllable at the end of the word was dropped off."
This is exactly what we did with the word 'do' in the above sentences.
Most such suffix vowels were gone by the Old English period, but their effects remained and in a few cases still do.
Hope it helps! :)