听力原文： M: Did you know that astronauts have made commercial products in space? W: No. I wasn't aware of that. What kind of products? M: Tiny plastic beads. W: Beads? Do you mean to tell me that astronauts have nothing better to do than make jewelry beads in space? It seems to me that they can make more useful things out there. M: Oh ! But these plastic beads aren't for jewelry. They can be used for many scientific purposes, from conducting cancer research to calibrating microscopes. W: That sounds better. But why make such objects in space rather than on earth? M: Because earth’s gravitational pull affects the beads. The beads produces on earth are distorted not exactly round. The ones made in space are precise spheres. What are the speakers mainly discussing?