It's rude to use your phone while having a conversation.
Mark is talking and eating with his mom at a restaurant when his phone dings.
Mark wants to check his email.
Mark tells his mom he needs to use the bathroom and checks his phone email there.
Mark pulls out his phone as his mom is talking and checks his email.
Mark's mom thinks Mark is polite and cares about her feelings.
Mark's mom gets upset and thinks Mark doesn't care about her.