The advice about "sun at noon" and "wristwatch" will give you the wrong locations for North, as others have pointed out, because of local vs apparent noon, time zones, daylight saving time, and so on. But here is a way to find North on a clear day (in the Northern Hemisphere):
On flat ground sink a 4-foot stick into the ground so it is vertical, and so its shadow will be unobstructed. Starting in mid-morning, place stones on the ground every so often at the point of the tip of the stick's shadow. The stone which is the shortest distance from the base of the stick points North (draw a pointer from the base of the stick through that stone). Note: you can stop placing stones when the shadow starts to lengthen.
The advice about "sun at noon" and "wristwatch" will give you the wrong locations for North, as others have pointed out, because of local vs apparent noon, time zones, daylight saving time, and so on. But here is a way to find North on a clear day (in the Northern Hemisphere):
On flat ground sink a 4-foot stick into the ground so it is vertical, and so its shadow will be unobstructed. Starting in mid-morning, place stones on the ground every so often at the point of the tip of the stick's shadow. The stone which is the shortest distance from the base of the stick points North (draw a pointer from the base of the stick through that stone). Note: you can stop placing stones when the shadow starts to lengthen.