An impractical joke is a practical joke that is, well, impractical, not easy, would have severe consequences, or generally a prank you'd like to perform but can't. These may include, but are not necessarily limited to, the following:

• Putting mass quantities of LSD in the Washington, DC water supply (or, alternatively, in the soup)
• Stealing someone's steering wheel
• Dyeing your town's river green
• Putting a kick me sticker on the President's back
• Changing the gravitational constant so your roommate falls when he/she gets out of bed in the morning
• Unleashing millions of flying cats upon your city
• Teleporting in and out of convenience stores to upset and confuse the clerks

The list goes on. You get the point.