OAK Holland Trading

and then this...

Solution:

Set one of the swithches to on. Wait some time. Set the first switch back to off and set another switch to on. Then go to the other room.

The bulb is on: it is the second switch (now on).

The bulb is out and cold: it is the third switch (not used).

The bulb is out and warm: it is the first switch (was on, now off).

There are two rooms.
In one room there are three switches, in the other room there is one filament lamp.
The lamp is connected to one of the switches only but the wiring is not visible in either room.
You can play with the switches as much as you want.
You can enter the other room to inspect the lamp only once!
How can you find out to which switch the bulb is connected?