Stuartnz, I agree with your comment that the teeth need to be a little bit apart, but it's a very small amount (1-2 mm). When I made my comment, I was thinking about the gap between the front teeth. Because those teeth can overlap without touching, they can be together (front-to-back) while no teeth touch.
In response to Literal's question about where the tongue touches, it touches at the inner gum line of both bicuspid teeth (the teeth with 2 points each). If you feel the inside of your upper teeth, as you move from back to front, you'll notice that there's a drop-off where the bicuspids end and the cuspids begin. When you have your teeth nearly closed, you can feel that drop-off with your tongue. The tongue should not be able to feel the inside corners of your teeth; it should be relaxed more and pushed between the teeth at the gum line. While keeping the tongue between the teeth and not at the corners of the teeth, shape your tongue so that there's an air gap for you to breathe. You won't be able to make a very large gap, but it's enough that if you plug your nose, you can breathe through it. Now, while keeping the sides of your tongue still, you can move the tip of your tongue to close the gap and shut off air flow. If you do it quickly while exhaling, it will make a sound like like a drum cymbal.
This is also the answer to Anuragz's question. The "drum cymbal" effect is how you make the /ts/ sound in "pots". The word has a moment of silence between the t and the s. It's that moment when the tip of your tongue is blocking the air flow for the /t/ sound at the alveolar ridge (for the upper teeth) and the sides of your tongue are moved between the upper bicuspid teeth at the gum line. Once your tongue is in position, you relax only the tip of your tongue to let air begin to flow again. The only difference between "pots" and "pods" is whether your vocal cords are moving during the sound. The tongue doesn't have to move between two different positions on the alveolar ridge to move between these two sounds; the tip only has to relax to allow air to pass.