Write down what you think happens. We stamp it, lock it forever, and email it back on the day. Say it in public and the doubters have to answer for it.

A prediction

Something about the world, with a date on it. Say it in public and the people who doubted you have to answer when it lands.

A promise

Something you will do, made to somebody in particular. Send them the link and they say whether you kept it. You do not get a vote.