Three assumptions

Engineers make 3 assumptions
about masonry as material.
This is a silly, bold presumption
but its use is all too real.

In compression it’s infinitely strong,
second it has no tensile strength,
third: blocks never slide; all wrong!
These three assumptions show the length

to which engineers must go
to see what masonry is today:
that it’s quite strong and it’s slow
to rot or rust from cruel decay.