Angus is in the front hallway of our house, nose to the ground like a bloodhound, sniffing like mad. He sniffs along the rug and then sticks his head inside a cardboard box and sniffs that. Then he moves on, sniffing in the corner behind the front door (oh, man, I must dust!), sniffing the bookshelf, sniffing the top of the radiator. He is as methodical as a detective.
Then he heads up the stairs.
On the landing, he first sniffs the windowsill, then each step, edge to edge. He stops. He looks at me. Found it!
"Yes!" I yell. He paws at the spot between the spindles of the banister where I have wedged a small metal box. It took him about two minutes of vigorous sniffing to find.
"Yes!" I say again, and he races down the stairs to me, tail wagging. I shower him with treats. And then I hide it again.
We have been playing this game — "nose work," it's called — for only a few weeks but Angus caught on fast. The object is simple: I hide a small perforated box that contains half a Q-tip sprinkled with scented oil, and Angus finds it by following his nose.

It's a game to him, and a fun one — he particularly loves the shower of treats every time he finds the box — but it's also great exercise for his brain and, really, for the rest of him too.
Dogs learn about the world through their noses. Their epithelium (nasal tissue) is 30 times larger than humans', and way more powerful — they have hundreds of millions of olfactory neurons, while we have a paltry 12 million.