DNA explainer: Why original copy of Constitution is kept in a gas-filled box?

There's one question that frequently pops up - where is the original copy of our Constitution? How many copies does it have and how are they kept?

from Latest News https://www.dnaindia.com/india/report-dna-explainer-republic-day-parade-why-original-copy-of-constitution-of-india-is-kept-in-a-gas-filled-box-2870499