Given a scientific question, generate a correct answer to it.

Input: Consider Input: On the ventral side of the embryonic disc, opposite the amnion, cells in the lower layer of the embryonic disk (the hypoblast) extend into the blastocyst cavity and form this?

Output: yolk sac

Input: Consider Input: What amazing machines smash particles that are smaller than atoms into each other head-on?

Output: particle accelerators

Input: Consider Input: What is the minimum mass capable of supporting sustained fission called?
Output: critical mass