Question

why conch come up from bottom of rice sack

Answer 1

Empty shells cannot rise up on their own without any force . We have three conchs at home and I tried the trick. They didn’t rise up even after one hour.

I think the conch shell they used in the video has some living animal like a crab inside it. Hermit crabs make empty shells their homes. That crab made the conch rise up. They don't tell you that, do they? :)