Before I start this post, I had better preface it with an explanation of WHY I am attempting to explain confusing things to students learning English. At the moment, I am teaching ESOL to two teenagers from Korea, a thirteen year old boy, and his fourteen year old sister. Overall, they are pretty cool and great fun to teach. But sometimes we run into problems. Some English words and concepts are surprisingly difficult to explain!
Gnome.
One day last week, my students and I were working through one of the books they are using for reading comprehension, when the following sentence appeared: "How was my brother to know that the gnome was a present?". Casting aside the weirdness of one’s brother receiving a gnome as a present to begin with, Master 13 immediately asked me what a gnome was.
I was surprised to discover that although I knew perfectly well what a gnome was, I could not explain it. The best description I could think of was "A short fat man statue"! Accurate though it was, it was not helpful, and I was left resorting to Google Images to help me out.
Unfortunately, upon searching for "gnome" on the computer, we were presented with a picture of a gnome in a Santa suit! When they saw this, they immediately exclaimed "Ah! Santa Claus! Gnome is Santa Claus!" Well yes. But no. Oh dear.
In the end I found some more traditional pictures of gnomes, and managed to explain that they are a kind of fairytale character and that some people like to put them in their gardens, which I THINK made a bit more sense. It seems that they don’t have gnomes in Korea! Perhaps I should advise them to buy one to take back with them when they go home! 
Principal.
I have to admit that I was very surprised by how difficult this was to explain.
Surely the school they go to at home must have a principal, or head teacher of some kind? It appears not, because when I said that the principal is the "teacher in charge of the school" and "the most important teacher", they still looked confused.
Unfortunately, this isn’t a case where a picture or photo is terribly helpful, and the place I teach at doesn’t really HAVE a principal that I can refer to, more of a supervisor/owner, which doesn’t really help me very much either. I’m still not sure that they understand this particular concept, so hopefully I have an epiphany soon over how to put it across! :/

Submarine.
This was another word randomly generated for creative writing by the computer system, and it another one of those things that everybody knows OF, but nobody knows how to EXPLAIN. Very frustrating.
The first thing I said was "It’s a ship that goes under the water". FAIL @ me. Unsurprisingly in hindsight, they thought I was describing a shipwreck!
This was another one I ended up using Google for (thank goodness for internet access!) and they did understand what I meant as soon as they saw a photo. :)

Echidna.
Thankfully I saw the questions over this one coming, and prepared myself with the photo on the right (which my supervisor printed out for me with much amusement!) before we got to the page in the book that mentioned this odd little animal!
Thanks to the photo, this wasn’t really terribly difficult to explain, but I’m including it in this list anyway, because I think it’s strange that it even came up as a word in an ESOL book (though the book I was using is published in Australia and the page we were working on included several Australian animals to demonstrate prepositions "The koala is IN the tree." "The echidna is ON the hill" etc…) and because the photo is extremely cute!
Platypus.
Yup. You guessed it. Another feature of the Australian Animals Prepositions Page. I showed them a picture of this creature too, and their reaction was "Is it REAL?"
Yes. Yes it is. HAHAH. I think if there was a competition for The Strangest Thing These Students Learned in NZ, the existance of the platypus might just win!