Boom Scholar

Creature — Goblin Advisor

Exhaust abilities of other permanents you control cost less to activate.

Exhaust — : Creatures and Vehicles you control gain trample until end of turn. Put two +1/+1 counters on this creature. (Activate each exhaust ability only once.)

Boom Scholar’s first ability can cause the mana component of the cost to activate an ability to be {0}.
Exhaust abilities can be activated any time you could normally activate an ability.
If an exhaust ability of a permanent is activated, and then that permanent leaves the battlefield and returns to the battlefield, it becomes a new object so its exhaust ability can be activated again.
If an ability triggers whenever you activate an exhaust ability, that ability resolves before the exhaust ability resolves.