In the next Bloons Tower Defense game, not all upgrades will be unlocked by achieving certain ranks or using certain towers or upgrades enough times. Instead, some upgrades will be unlocked by sending your monkeys to the Blooniversity, where they will learn the new tactics for defeating the bloons. You can only send one monkey at a time, each upgrade costs a different amount of Monkey Money (more powerful ones cost more), and you must wait a certain amount of time before the monkey will be back (if you start a game during this time, the tower you sent to the Blooniversity will be unavailable). The amount of time it takes for the monkey to learn the upgrade and get back depends on how powerful the upgrade is.