For April Fools at a school I worked for, I printed off one of those "Your printer has been upgraded to be voice activated" signs and put it on their fairly new printer, went into the room opposite and waited. I heard far far too many teachers saying "Copy!" repeatedly, getting more and more agitated before 'manually' copying their document. I had to come clean before I left at lunch time because one of the teachers got quite angry at the prospect of a voice activated photocopier and refused to use it that way. The following year April Fools was on a school holiday so I couldn't pull it in the school I was in that day but i fully expect it to work somewhere else.