What letters are sometimes vowels?

This is a bit of a trick question! It depends on what you mean by "sometimes vowels."

Here's a breakdown:

* Traditional vowels: The letters traditionally considered vowels are A, E, I, O, U. These are always vowels in English.

* Y and W: The letters Y and W can act as vowels in some situations. This happens when they're the main vowel sound in a word (like in "fly" and "how"). However, they can also be consonants (like in "yellow" and "winter").

* Other letters: Technically, any letter could be considered a "vowel" if it represents a vowel sound. For example, the letter A in "father" makes a different sound than the A in "hat," and it sounds more like the vowel in "father" might be ah. However, this is a nuanced view, and the usual categories of vowels are still the most useful for understanding the English language.

So, the answer depends on how strictly you define "vowel." But in general, the letters Y and W are the ones that are "sometimes vowels."