Go in one ear and out the other
Definition 1 of 1
Pro Tip 1/3Nonliteral Phrase
Understand that this phrase is not meant to be taken literally, it's an idiomatic expression. 
Even after explaining the math problem, it seemed to go in one ear and out the other.
Pro Tip 2/3Selective Attention
Use it when someone is not focused on the message and they ignore it. 
During safety training, his questions about breaks made it clear the instructions went in one ear and out the other.
Pro Tip 3/3Inattentiveness Consequence
Implies that the listener won't act on or remember the information due to a lack of focus. 
I reminded you to buy milk, but it must have gone in one ear and out the other.