My son (who's three) has come home from nursery humming a tune which I identify as 'Alice the Camel' - but he says it's not that. When I asked him what the words were, he said something that sounded like 'Cam by car stigger', and then 'Chug chug chug chug chugging along'. The only thing I can think of that might make sense is 'Combine Harvester', but that doesn't really help me much.

Anyone have any idea what he might be talking about?