Wet Grass

To avoid the icy roads, Holmes moves to LA; Watson moves in next door.
One morning as Holmes leaves for work, he notices that his grass is wet. He wonders whether he has left his sprinkler on, or it has rained.
Glancing over to Watson’s lawn he notices that it is also wet.
Holmes thinks: Since Watson’s lawn is wet, it probably rained last night.
He then thinks: If it rained then that explains why my lawn is wet, so probably the sprinkler is off.