If, for example, you're hiking in the vicinity of 37 degrees and 21.938 minutes north latitude and 122 degrees 13.009 minutes west longitude — a woodland in Portola Valley — all you have to do is look around.
But first you'll have to cross Corte Madera Creek via a fallen tree, a bit daunting in spring when the creek is running. And there is poison oak to watch out for. It will have helped to have decoded the following online hint: "Just above the highest flood line under a log that is held upslope by a standing tree."
Find the jar and you will have accomplished a task rated at two and a half stars on a five-star scale. This is geo-caching, a planet-spanning game of hide and seek.
Enter your name and maybe a message in Mr. Troll's log book, pick out a gift (if there are any) such as a rock or a set of beads, leave a similar gift if you have one, and you can notch your belt as to this cache, one of 369 listed within a five-mile radius of Portola Valley and 77,686 in California.
This reporter accompanied Portola Valley resident Danna Breen and Los Trancos Woods resident Charlie Adams on a recent fine spring morning in search of three geo-caches within walking distance of Ms. Breen's home on Alpine Road.
In the two and a half hours it took to find them using a handheld $100 global-positioning device, the three hikers experienced a vigorous workout, plus walks in several woods, a stunning view of Windy Hill, and a quiet amble across a lovely meadow.
"The people (who) hide these things usually hide them in places they find interesting. They're so far off the beaten path that you'd never come across them (otherwise)," said Mr. Adams, who has searched for geo-caches in Ireland and plans to do more of it in France soon.
In one search in Ireland, Mr. Adams, as instructed by the online hint, looked for clues on a gravestone, he said. In another closer to home, the cache was a lipstick case that had been placed inside a drilled-out horse chestnut and hung like an ornament on a horse chestnut tree amid many other horse chestnuts.
These are treasure chests, said Ms. Breen, who left a tiny piece of old tile collected from Italy's Amalfi Coast in a cache along the Sweet Springs trail. "I just love it," she said.
On this morning, Ms. Breen ran into Waltraut Monroe, a Portola Valley resident out geo-caching with her friend Ursula Berg and Ms. Berg's grandson Matthew, who had scored a plastic Power Ranger figure and a finger puppet.
"It's very magic," Ms. Berg said. "They're hidden all over the place."
Geo-caching is, as its official Web site says: "The sport where you are the search engine." For more, go to geocaching.com.