I used to watch the chub from a bridge on the River Leam from a place I couln't fish.
One morning I took a slice of bread with me to the bridge and deposited a few pieces of bread flake in the water. The smaller chub took these no problem at all.
Then a big old chub arrived on the scene.
The smaller chub seemed to part to let the old guy get to the bait. This big chub, which I put down at 5 1/2lb at least, picked up a piece of flake in its lips and swam downstream for at least 15 yards with the flake hanging on the outside of his mouth, The water was crystal clear, I had my polarised specs on and I could see everything. He then proceeded to shake the lump of flake about until several small pieces came off. He then dropped the large lump, mouthed a few of the tiny bits and then set off downstream as though terrified, making a bow wave in the water.
I am sure that old chub was making sure there was no hook attached to the bait, and even then there was something that scared him.
My word, anyone who thinks fish are stupid, haven't either been fishing or have never taken the trouble to watch them.