I always keep the little Sony RX100 in my jacket pocket whenever I walk around the city. You never know what you might see.
This image is an “almost”, not quite skewering that Cartier-Bresson-ish “decisive moment” street photogs yearn for. I only had one chance to get it correct in the frame as he ran through and I missed. If the paper airplane were more distinguishable–possibly a faster shutter speed?–that might have helped. If there were better separation of the boy and the leaves, that would have helped as well–maybe the leaves in front of the boy and the boy just entering the frame?
Also, if he were running through the frame of that square window to his right and down below, might that have been better?
Another idea might be to simply retitle the image, “Boy Chasing the White Dove of Peace”, or some pseudo-arty-farty shenanigans like that. (Since you can’t readily tell it is a paper airplane in his hand, and it looks more like a bird anyway.)
Ah, well. All you can do is stay alert and keep trying!
Near that same area of Barcelona, I came across a very different scene that made for what I thought was an OK abstract. Hopefully, it will disorient you for a few seconds before you figure it out…or, perhaps, it is obvious: