Why multiplying the numerator and denominator of a fraction by the same number results in an equivalent fraction?

Im Not That sure But I Belive That If Denominators Are The Same Keep It And Just Do The Math In The Numerator's. (For Adding) But Multiplying You Do Both and If The Numerator Is The Bigger Than The Denominator Your Going To Have To Do Extra Math Like A Common Denominator (I Belive).