instruction:
Given a scientific question, generate a correct answer to it.
question:
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?
answer:
yolk sac

question:
Atoms of different elements can combine in simple whole number ratios to form what?
answer:
chemical compounds

question:
The ability of a water spider to walk across a body of water is an example of what?
answer:
surface tension