The special ability of a cleric to drop a prepared spell (but not a domain spell) to gain a cure or inflict spell of the same level or lower, or of a druid to drop a prepared spell to gain a summon nature's ally spell of the same level or lower. Since the substitution of spells occurs on the spur of the moment, clerics need not prepare their cure or inflict spells in advance, nor do druids need to prepare their summon nature's ally spells in advance.