Historical Rules · 16 July 2010
Rule 702.30a
Official source text from the 16 July 2010 Rules release
Kicker is a static ability that functions while the spell with kicker is on the stack. "Kicker [cost]" means "You may pay an additional [cost] as you cast this spell." Paying a spell's kicker cost(s) follows the rules for paying additional costs in rules 601.2b and 601.2e-g.