What is a simple explanation for how the number of bitcoins in circulation is determined? Often times when I explain to someone that there is a limited supply of bitcoins available that need to be divided across all the people who want to have bitcoins, the people who are computer illiterate will ask how I know that the number of bitcoins in circulation can not be changed or manipulated. They usually just get a glossed over look on their face when I start talking about decentralized protocols. Has anyone come up with a good way of phrasing it or an good analogy to get people to understand how these limits are determined, enforced, and protected?
You could explain the coin limit like this:

When Bitcoin started, its creator set a rule - "No more than 21 million Bitcoins will ever be created, and they will be given out gradually for creating blocks" (Add further explanation of block reward or proof-of-work here if needed). Everyone who joined Bitcoin knew about this rule, and they were all for it. As everyone can see all the money created by solving the blocks, everyone can check if someone tried to go against the rule and cheat. If cheating is detected, nobody will accept the solution, instead looking for one that follows the rule. Because everyone is checking everything, nobody can cheat and create more Bitcoins than is allowed.