Lantern Spirit has an activated ability that allows you to pay one blue mana and return the Spirit to your hand. This ability is obviously useful as it helps the Spirit dodge removal spells or block and not take any damage.

It is important to note that returning the spirit to your hand it part of the effect of this ability and not part of the cost. You can tell as the text “Return Lantern Spirit to its owner’s hand” appears after the colon. Because of this, players can respond to the ability and perhaps kill the Spirit before the ability resolves, thereby preventing the Lantern Spirit from returning to its owner’s hand.

For example, let’s assume that a Lantern Spirit is blocking an attacking Riot Devils, and the controller of the Lantern Spirit has only one blue mana available. If the controller of the Lantern Spirit activates its ability, his opponent could cast Geistflame and deal lethal damage to the Spirit before that ability resolves. This would cause the Lantern Spirit to go to the graveyard and stay there. The Lantern Spirit’s ability will still resolve, but it will do nothing. The Lantern Spirit card in the graveyard is a ‘new’ object not connected to the Lantern Spirit that was in play, so it will not be returned to its owner’s hand.