Wave-Particle Duality: Where's the Line?

So, wave or particle? Is it like, water or... or... a thing that's not water but maybe sorta kinda water? Ugh, my head hurts.

I mean, take this coin:

Is it a wave when it's in flight, and a particle when it's still?

Or is it like, a thing that's both at the same time? Like, can you be in love with two people at once? Asking for a friend.

Help me, Google!