Can two light rays collide?

Is there such a thing as a true circle? Can two light rays collide with one another? What makes the sky look blue? Have aliens ever visited Earth? Will time travel ever be possible? What's the perfect pH for seed germination? Would a truck full of birds weigh less if all the birds took flight? What's at the bottom of a black hole? Why is everyone's poo brown despite the fact we all eat different foods? This week the questions for Dr Chris come from the kids at Gaia Waldorf School, and they're very hard!