"Mary Fisher" wrote
| Are modern doors really made of cardboard?

Yes :-) (Or something like it)

Two flat outer skins of hardboard separated by a cellular
corrugated-cardboard core. (Cardboard has quite good compression strength
provided it's supported from bending - think of the weight a toilet-roll
core will support on end.)

There was an episode of the Generation Game where the contestants had to
make a door.

Owain