Sparks, explained

That is the whole conversion, and it is deliberately the simplest one possible: 300 sparks is ₹300, a change at 5 sparks cost ₹5, and you can check any figure on your bill in your head.

The unit exists for one reason. You must never have to meet tokens, models or what a change cost us versus what it cost you — but "your balance is ₹287.43" invites "₹287.43 of what?", and the honest answer to that involves all three. A spark is a name for your own money that does not drag the machinery in with it.

The rounding rules, in your favour twice

• A balance rounds down. You are never shown sparks you cannot spend.
• A charge rounds up, and never to zero. A list of "0 sparks" would read as free, which is the one impression a bill must never give.

Running out

At zero the builder refuses new changes. A change already underway always finishes — the check happens before a change starts, never during. Admins are exempt, because locking the operator out during an incident makes a bad day worse.

The sentence you were about to send is held and offered back. It is never run for you when money arrives.