Energy Tap

Turning creatures into mana was a question the game had not fully answered yet, and this is among the rawer first stabs at it. The conversion is the design knot: you pay a blue mana to tap a creature and collect colorless equal to its mana value, so the spell only nets you mana when that creature is expensive enough to clear the one you spent casting it. Tap a one-drop and you break even; tap a four-drop and you are up three colorless for a turn. The arithmetic only works on a fattie you somehow already have in play, and even then at a cost the card hides: because Energy Tap demands an untapped target, it locks that creature out of attacking or using its own tap ability that turn. You are spending the body's whole turn to flatten its cost into raw colorless mana, a trade blue historically had to make where green simply ramps for free. The follow-on lineage shows up in everything from the "tap big things for mana" school to the later crop of creature-tappers that feed storm counts and cost-reduction triggers. The limits are baked into the era: sorcery speed and a strict colorless output, where later designers learned that the interesting payoff is mana matching the creature's color rather than washing out to generic-only, and that the effect wants to fuel something at instant speed rather than sit as a clunky main-phase ritual.