Eros challenged him to see how far he could throw a grape, and somehow it had turned into a competition; the two taking turns to lob grapes across the courtyard from where they sit perched on a wall. He's actually doing well and feeling confident, right up until Heracles crosses into the projectile path of a grape Gany has thrown particularly hard. It hits him right on the top of his head, and Eros promptly bursts into laughter while the cupbearer looks on in horror. "S-Sorry!"
The hero had been lost in a world of his own, feet tracing their way through the halls of Olympus by memory. And he would have remained lost in those thoughts, had he not had the misfortune to be crossing the courtyard on his way to find his wife in the gardens. The impact of something bouncing off his skull pulls him back into the present moment and he looks down reflectively, eyes roaming the stones until they find the offending object. He knows immediately from the laughter and the sheer horror on Ganymede's face that whatever the pair is up to was not the cupbearer's idea, and so he turns his attention to Eros, fingers easily crushing the grape as he speaks.
❝I wonder how far I could throw you, Eros. I would wager I could make it at least to the gardens from where you're standing.❞ His attention shifts to Ganymede, head tilting in question. ❝What do you think, cupbearer? Shall we find out how far a god can fly?❞