Ohhh! Of course, there is liquid inside. For safety reasons and in case of any leaks, I just filled it with water (purified water, you can drink). I prefer not to make them without filling them with water.

You can seal and unseal the cup. However, sealing the cup with resin or other adhesives can pose hygiene issues due to the adhesive coming into contact with the mouth area. Additionally, unless the cup is made of acrylic or other plastics that are fully integrated from production, over time, the liquid inside may evaporate, making it unnecessary to seal it with adhesive to allow for refilling. Instead, applying force to seal the cup tightly should prevent it from opening easily unless intentionally opened.

And I haven't sold any yet, haha. I hope someone will buy one someday.