Leaving the ad to the user

Most free apps show ads without asking. That is selling attention without the product’s permission.

Another way

The ad opens only when the user asks for it, and earns permanent points in return. Points never expire; they unlock whatever feature the user wants, whenever they want.

There is a cost: revenue becomes unpredictable. But the user is not cursing at the product while using it, and we believe that buys retention.