My dog has a squishy bump that doesnt bother him?

i had just noticed that my chihuahua mix appears to have a bump on his rib cage. It's squishy and feels like there is water inside. I ume it is on top of his skin because when i stretch his skin one way, it moves with it. also, he is not irritated by it and doesnt even notice it. im afraid it might be a problem anyway. what is it?!