A rope-clock

[Genady]

You have a rope and a lighter. If you light a rope, it burns about 1 m in 1 hour. However, it does not burn at a uniform rate, e.g., not necessarily 0.5 m in 0.5 hour.

Can you measure 0.5 hour? 15 minutes? 20 minutes?

[Genady]

Perhaps a better title would be, a rope-timer.

[Genady]

Here is how one could measure 30 minutes with the rope.

Spoiler

If one lights the rope in two points 1 m apart simultaneously, the flames meet in half an hour.