When I was an apprentice one of our favourite pranks for any pipesmoker that left his pipe unattended at work was to remove the contents and put 4 or 5 red topped match heads in the bottom and re fill.

Used to cause us much amusement when the bowl burned to matchead level - used to make a nice little volcanic display.

No one ever knew it was us, and one guy used to just nochantly carry on puffing away post volcanic erruption, whilst the others would put it down in a hurry.

Bloody kids.