At one end of the room put a single table with a small pot on
it. Across the middle of the room put a row of tables. On one
table put a "spot". This can be a piece of paper taped down
on the table with a big black spot on it, or whatever.
Table with pot
Barrier across room
Table with spot
Without crossing the row of tables, find the distance from the spot to the pot. You can use anything you would typically find in a house. Things like string, pencil, paper, ruler, tape measure, etc. What you can't use are things like theodolites, sextants, or other exotic devices. The exercise is perfectly possible with a ruler, some string, paper, pencil, and a calculator. With care, the calculator isn't necessary.
Before giving a "solution" I'd like to invite people to suggest ideas for solving this challenge. Edit this page to add your comments and ideas.
It's fairly well-known how to find the height of a tree on a sunny day.