10 Things I learned from traveling to more than 19 countries
Branched from @younes, though I only have 19 (I might have missed one) to his 40.
1. The sky is always blue
I've heard people comment that the sky looks brighter or different in some places, but I take comfort in knowing I'm under the same sky on the same planet no matter how different everything else.
2. Strangers don't want to offer you things for free, they actually want something from you.
My mother-in-law just encountered a clown in downtown Berlin who made her a balloon animal and she was enchanted; till she figured out the clown wanted a donation and wasn't just trying to be charming. I feel like she's old enough and well traveled enough to know better...
3. "The system" may be exploitive to tourists, but it's "the system" for a reason.
In Budapest, a friend didn't want to stand in line and pay exorbitant exchange rates to get local cash. A man approached him on the street and my friend counted the cash very carefully and it wasn't counterfeit from what we could tell. Suddenly the guy says "the cops!" (it was illegal to change cash outside the official exchanges), and pulls a switcheroo with the wad of cash. We tried to give chase but the guy had an escape route through a store and out the back door. My friend lost 90% of the money he had on him trying to save like 5-10%.
4. Fight Jet Lag Tooth and Nail
I don't have any special tricks, I just know acknowledging that you're tired is OK, but continually calculating what time it is back home will only keep you mentally on that clock. Do everything you can (alarms, dragging yourself out of bed) to get on the local time as soon as you can.
5. Try local food
You travel with all 5 senses, but we mostly think about 'sightseeing'. Treat your sense of taste and smell to the differences.
6. Work like they work
If your travels are work/business related, observe how people work there. Do they take 90-minute lunches? Is there a formal address you should be using in a professional environment? Are business hours strictly observed (e.g. France has a no-work email after-hours law)?
7. Listen to the music
See #5. Live music can literally make a trip worthwhile in and of itself.
8. Smiling is universal
Comfort in a different culture (similar to #1)
9. You will not have the comforts of home
Homesickness is natural, and you may miss your bed, your shower or whatever, you need to focus on the positive - you won't have all the same problems as back home and you will have experiences you couldn't have back home either.
10. Look for common threads
If you've been to enough places you might see, for example, a dish that is similar to one they serve in a completely different culture/cuisine. Both Mayans and Egyptians built pyramids - finding these human universalities is one of the great joys of travel.