What functions similar to a relay race?

I'm trying to find examples of things which work together in pairs, one after the other, like a wrestling tag team, or a drop creating ripples whether it is mechanical, natural whatever. Show me what you got, I'm a bit brain dead at the moment.|||a vodka and tonic; a cat and mouse; a wild wind and a windmill; a halo and a saint; a bacon and an egg .... getting ideas yet? O.K. I'll ramble on - a watermill and water? A carver and a knife? A sunset and the position of the sun? A fire and warmth ...|||Politicians passing the buck