iPhone to iPhone text messages
My wife and I both have iPhones: an iPhone 5 and an iPhone 6.
Occasionally, in the middle of exchanging a series of text message with another iPhone user, we will notice that now and again the other party's text bubble will show as green, not blue.
We understand that iPhone to iPhone texts go via Messages and are free and have a blue bubble whereas texts to non-iPhone are not free and show a green bubble.
What gives with the green bubble in iPhone to iPhone text exchanges in the same "conversation". Why do the text bubbles switch from green to blue and back again?