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A British couple believes they've come across a hot cross bun that was baked more than 200 years ago. Host Scott Simon explains.

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IRA FLATOW, HOST:

You're listening to SCIENCE FRIDAY. I'm Ira Flatow. And for the rest of the hour we're going to talk about collisions, space collisions, space impacts, with Erik Asphaug, who's Ronald Greeley chair of planetary geology, School of Earth and Space Exploration at Arizona State University. Welcome to SCIENCE FRIDAY.

ERIK ASPHAUG: Thanks very much, Ira.

FLATOW: You must be very busy since this last collision in Russia of this asteroid.

When does a story about science become science fiction? Cosmologist Lawrence Krauss and theoretical physicist Brian Greene discuss how to spin a yarn about string theory or the Big Bang, without hyping the science. And novelist Ian McEwan, whose books touch on neurosurgery and quantum field theory, talks about what science offers to fiction.

The Sonoran Desert, which spans some 100,000 square miles in southwestern North America, is one of the most diverse desert ecosystems in the world. Host Ira Flatow and guests discuss some lesser known desert creatures, and explore the secret life of that American southwest icon, the saguaro cactus.