April Fool's Day is coming up. What's the best prank you ever pulled?

My kids are at the age (8 & 4) where they're expecting me to pull some sort of elaborate April Fool's Day Prank. Sure I could put green food coloring in a jug of milk. Or put clear nail polish on a bar of soap. But I want something that my girls will recall when they grow up and have kids of their own. Any ideas?

Speaking of pranks, let's keep them safe. Not like these stupid guys in northern Idaho who are stretching plastic wrap across I-90. It's already caused one car's windshield to blow out. They're lucky nobody was killed! You can read the whole story HERE.