This is what they call an IED -- an abbreviation for Improvised Explosive Device.
 

One of the tricks of the trade our guys are finding in Iraq.  Notice it is set so as the wheel turns it will pull the pin and causing the grenade to detonate between the rear of the vehicle and the passenger compartment.
 
Take a close look.  This is what you are looking for when we say search vehicles for small improvised explosive devices.  How many of you would notice the wire tie on the wheel?