Why are most sent texts SMS but some MMS?

[AC Question]

Occasionally (and apparently randomly) my texts are sent MMS rather than SMS, and therefore incur a charge. Why is this and how can I know before I send it what I am doing wrong? There is no obvious reason; some long texts with emoticons go as SMS whereas a couple have been MMS.

 

[belodion]

Do the MMS ones have an emoticon in common, not used in the SMS ones?