Prick the ears up - Literally it means to stand up one's ears. But the actual meaning of this idiom is to start listening carefully to something or someone just in order to listen something related to their benefits or something of their interest and betterment.
It is an informal piece of idiom which we often use, because it is a modern one.
1. Listen to what she is saying, be ready and prick your ears up.
2. Everyone had pricked their ears up when the PM was delivering the speech.
3. Always prick your ears up when the teachers explain in the class.
4. While they were chatting, I had pricked my ears up but I could not listen anything.
5. My neighbors always prick their ears up in order to listen each others' gossips.