Why fish put the small in their mouth?

It sounds like you might be asking about the saying "a fish can't see the water it swims in." This is a metaphor, not a literal statement about fish putting anything in their mouths.

Here's what the saying means:

* Fish live in water. They are surrounded by it, so they never experience what it's like to be outside of water.

* We are often unaware of our own environment. Just like the fish, we can be oblivious to the things that are most familiar to us, such as our own beliefs, habits, or the world around us. We may not see them objectively until we experience something different.

So, the saying is about our own perceptions, not about fish literally putting things in their mouths.