This remains my favourite ever joke. I hope your groan reflex is in working order.
It is 1895. The British Empire spans the globe, and Victoria reigns over all from London.
In the Austrailan outback our heroes, Charles Carruthers and Bernard Fortescue-Smythe, make plans for a daring journey. They intend to walk across the Nullaboor Plain. They have devised a cunning way to store enough water, and have the proper gear.
They set off.
Part of the way across, with the sun beating down from a cloudless sky, the worst happens. The water container bursts, and the entirety of their water supply soaks into the parched earth. They are almost exactly in the middle of the Plain (they think) and all directions are alike. They press onward.
By the end of the day they are feeling dizzy through lack of water. They rest briefly then press on, hoping to find some shade for the day.
When the sun rises they are in the middle of an utterly flat, featureless area. There is less shade than there is water - and there is none of the latter. The sun hammers at them, draining them of their reserves. Despite all their skills, they begin to wander, being so groggy in the heat.
The next morning sees them still walking, still looking for water or shade - anything. By midday they are reduced to crawling.
They can’t estimate how long has passed before Carruthers (who is in the lead) realises he has gone blind. He stops, and gropes around. No, not blind - he can see the glare of the Sun on the ground behind him, but something blocks it. He croaks “Shade…” Both he and Fortescue-Smythe pause briefly before resuming their crawl.
They don’t get far before Carruthers comes to a stop - there is something in his way. As he realises this and begins to lift his head, the obstacle disappears, to be replaced by a pair of feet - dark-skinned feet.
A voice cries out “Visitors!”
Carruthers and Fortescue-Smythe find themselves surrounded, lifted up and carried into the hut. They lift their heads to see a young Aborigine, who smiles at them and, in impeccable Queens English, welcomes them to Mercy and asks what they want to drink. Years of ingrained habit cut in, and they ask for tea. The man calls out a sharp command; “Tea for our guests!” Feet scurry to obey.
Being thoughtful, the natives provide the explorers with a glass of water each “to tide you over.” They learn that their host (who’s name was unpronounceable) went to school at Eton and Jesus College, Cambridge (where “everybody called me Sin.”) They reminisce about schooldays until finally the tea is served.
Sin pours. As he does so Fortescue-Smythe asks “What kind of tea is this, Sin old fellow? Chinese, Assan?”
Carruthers leans across before Sin can answer and saays “It does not matter old bean. Tea is tea, after all.” Fortescue-Smythe  acquiesces weakly.
Milk and sugar having been added to taste, the explorers get their tea. Carruthers stirs briefly and is just about to take his first mouthful when Fortescue-Smythe  cries out. “Here. This tea is lumpy! What is it made from?”
Sin answers, a touch reluctantly, “It is made from the pressed flesh of koala bears.”
“Koala bears? Koala bears? But… but… don’t you even get the lumps out?” Fortescue-Smythe gasps.
Sin looks a little surprised. “But of course not. Everyone in England knows that the koala tea of Mercy is not strained.”