Please keep in mind that it's essential to consult with a healthcare professional if you have any concerns about your health.
The hollow sound you hear when tapping on your head and making an "o" shape with your mouth is likely caused by the resonance of the sound waves within your oral cavity and the shape of your mouth. This phenomenon can occur when you create a specific space with your mouth, which allows the sound to echo or resonate, leading to a hollow sound perception.
It's important to note that the sound you hear does not indicate that your head is empty or that there is anything wrong with your brain. The human skull is a protective casing for the brain and is not hollow. The brain is surrounded by cerebrospinal fluid and other structures that provide support and protection.
If you are experiencing any unusual symptoms, such as headaches, dizziness, memory problems, or any other concerns related to your head or brain, it's crucial to consult a healthcare professional for a proper evaluation. They can assess your condition, conduct any necessary tests, and provide you with accurate information and guidance based on your specific situation. It's always better to be safe and have any health concerns addressed by a medical expert.