Vomiting 24 hours apart but no other symptoms

[llareggub]

For two nights running my 16 month old was sick at around 11pm. Now, this morning, my older son has been sick first thing in the morning for the second day running.

There are no other symptoms. They are both perfectly well in themselves: bowel movements normal, no fevers, both playing happily and normally.

Any ideas about what this could be?

Have had this on a number of occasions with dd. GP only said to keep fluids up and could not offer any diagnosis apart from that it would probably be a virus ...