How come we say 'mon amie' instead of 'ma amie'?

Usually, possessive adjectives agree with the noun that comes after them. For example, we would say 'mon frère' for 'my brother', but 'ma soeur' for 'my sister'. However, when a feminine word starts with a vowel, we change the adjective to masculine to make the whole thing sound smoother.It is a weird rule to learn, but it makes the pronunciation easier.