Note that Manifold is (from their “About” page):
Manifold is a play-money prediction market platform where you can bet on anything.
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• Mana (Ṁ) is the play-money used by to bet on our platform.
• All users start with Ṁ500 for free and can earn more by winning bets and gaining free bonuses.
• Most of our users never spend real money!
• It cannot be converted to cash, but can be redeemed for real charity donations at a rate of Ṁ100 to $1.
Thus, people betting are not actually backing their bet with money, so this is more like “putting your mouth where your mouth is”.