Seems that Herculaneum and Mercury gave Sparks a rather big cake for his birthday, though Arye, who was also present for the party was a bit suspicious due to Herc's history of pranking. Herc claimed that there was nothing up about what he and his brother did, meanwhile Mercury lifted Sparks up so he could get a good view of his cake.
(Actually the candle on the cake is a stick of dynamite painted pink, not that Herc or Merc are gonna blurt it out, at least before everyone gets covered in cake)