What is a window sill typically made of? I'm more interested in standard
construction rather than custom endangered species rainforest wood or
something like that.

I'm just about ready to start replacing my windows and I'm looking at the
window sill and framing material - 3/4" plywood. Seems pretty damn chintzy
to me, especially around something that will in all likelyhood get damp or
wet at some point. I'm personally not going to replace it with plywood, but
I wanted to level set my thinking here and make sure I'm not being overly
harsh on the contractor who built the place. 1960 is the build date for
this house.