There is a soft water-blister-like bump on the head; touching it causes no reaction in the cat

A water-blister-like bump on the head may be: 1. Lipoma, a common usually harmless skin tumor that is soft, moves under the skin, and causes no discomfort. 2. Abscess, a bacterial cyst that is red, swollen, and painful with pus if burst; have a vet clean and treat it.

3. Other benign tumors like adenoma or fibroma, usually harmless but sometimes needing treatment. 4. Skin infection causing hyperplasia, pus, or abscess. Whatever the cause, take the cat to a vet; an exam and possibly cytology or biopsy determine the nature and best plan.