Lisa throws a watch into her trash can and the police obtain a search warrant for it.

The police are investigating a burglary.  Of particular interest is a missing watch.  An officer happens to see the victim's neighbor, Lisa, throw a watch into the trash can next to her house.  "Stupid thing doesn't even work," Lisa shouts within earshot of the police.

An officer wants to examine the discarded watch, but reasonably concludes that it is within the curtilage of Lisa's house.  After all, the trash can sits next to Lisa's house behind a short picket fence, with a sign saying that the garbage is off limits to the public.  Rummaging through Lisa's trash now would most likely violate the Fourth Amendment.

Not willing to risk that any evidence will be suppressed under the exclusionary rule, the police officer obtains a search warrant.  In his affidavit, the officer attests to the following facts: a burglary took place, a watch was taken, and next day, the victim's neighbor placed a watch into her trash can while saying that it "doesn't even work."  Seeing probable cause that a crime was committed and that the evidence is in Lisa's trash can, a magistrate judge grants a warrant to search Lisa's trash can.  

The watch is the same one that was stolen from the victim's home.  The watch can now be admitted at trial against Lisa.