This was designed for use in the garden but as it's winter we tested it in the bathroom. It works!
True motion tracking is expensive and complicated so this system activates when the victim moves into range and randomises the position of the cannon and the firing time within a limited area. The sometimes fickle readings from the PIR sensor just adds to the cannon's randomness, and therefore the fun.
This is my first Arduino project.
Step 1: Parts
Windscreen washer pump
PIR motion detector
TIP 120 npn transistor
12v rechargeable NiMH battery
watertight box to house the workings
Various length 22 awg jumper leads
Short length of pipe