You are sitting in a quiet room when a clock on the wall starts ticking. Tick. Tock. Tick. Tock. It sounds loud, annoying and also impossible to ignore.
Ten minutes later, you are reading a book and then, the ticking has vanished. The clock didn't stop your brain simply erased it.
This is called:
"Sensory Adaptation."
Your brain receives millions of bits of information every single second from your eyes, skin, nose, and ears. If it paid attention to all of it, your mind would overload instantly. So, it uses a simple filter: it only cares about change.
When a sound, smell or physical feeling stays constant, your brain decides it is no longer a threat or a priority. It turns down the volume on that input to save energy for new information.
It's the exact same reason you don't feel the weight of your own clothes touching your skin right now, until someone mentions it. The moment you read those words, your brain reloaded the signal, and suddenly you feel your shirt against your shoulders again.