What is a Vain Word™?

Question

^{This is inspired by the What is a Word/Phrase™ puzzles originally created by JLee.}

If a word has a certain property, I call it a Vain Word™.

In each of the short, unrelated sentences below, only one of the choices is a Vain Word™.

The sentences are for context only, and do not affect whether a word is a Vain Word™.

1. I missed the bus, so I took a ( CAB / TAXI ) to work.
2. The old house down the street is covered in ( IVY / VINES ).
3. Having too much to drink can make you ( VOMIT / PUKE ).
4. To my dismay, a ( YOUNGSTER / YOUTH ) always loiters in front of my store.
5. Archaeologists who explore tombs may have to contend with a ( ZOMBIE / MUMMY ).
6. I noticed the ocean was very ( WAVY / CALM ) while I was sitting on the beach today.
7. Having been beaten by my enemy last time, this time I was prepared to ( OUTSMART / OUTWIT ) him.
8. The controversial social experiment turned out to be a ( HOAX / SHAM ).
9. The mathematician was stuck trying to prove something, until he recalled a rather obscure ( AXIOM / THEOREM ).
10. My friend showed my his ( GARGANTUAN / MAMMOTH ) truck. It was quite an eyesore.

What makes a word a Vain Word™?

Answer 1

Vain words are

made up of letters which are symmetrical about the Y-axis, presumably because they like to look at themselves in the mirror

That is:

1. TAXI
2. IVY
3. VOMIT
4. YOUTH
5. MUMMY
6. WAVY
7. OUTWIT
8. HOAX
9. AXIOM
10. MAMMOTH