Why does my text messages convert to MMS on my Samsung Galaxy A3 (2016)?

I generally use the Text function.  Why sometimes does this revert to mms?   I have seen my bills increase and when I query this, I am told it is because I have used mms. 

Hi there. I believe you go over 160 characters that's why your text message automatically converts to MMS. I suggest installing other text messaging app that supports more than 160 characters. Visit Google Play store to install one. Hope that helps.

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