This item striked my attention and seems to be a very good practical joke if you are bored some days. I myself would use it to scare the people walking around my block but using it for a scarecrow or yard watcher is ok too.

Here's how it works:
1) Connect the battery–operated device (top) to your hose
2) Stick it in the ground
3) Wait

When the built–in heat and motion detector senses movement —up to 35 feet in front and 45 feet to the sides throughout its 100° detection zone it shoots a 3–second, 2–cup water burst at the trespasser. It then resets in 7 seconds, "ready for the next intruder." "The effect is both startling and immediate!" No doubt. You get up to 5,000 bursts from a single 9-V battery. The device is 24" tall and covers a 1,000–square–foot area. Effective day and night.
More information - Here Ohgizmo - Here Product Page - Here