Mice have built an underground house consisting of chambers and tunnels:

- each tunnel leads from one chamber to another (i.e., none are dead ends),
- from each chamber, exactly three tunnels lead to three different chambers,
- from each chamber, one can reach any other chamber via tunnels,
- the house has exactly one tunnel such that its collapse would divide the house into two separate parts.

What is the minimum number of chambers the mouse house could have? Sketch how the chambers could be connected.

(K. Jasenčáková)

Hint. Start with the critical tunnel.