From an early novel called Roughing It:
Sage-brush is very fair fuel, but as a vegetable it is a distinguished failure. Nothing can abide the taste of it but the jackass and his illegitimate child the mule. But their testimony to its nutritiousness is worth nothing, for they will eat pine knots, or anthracite coal, or brass filings, or lead pipe, or old bottles, or anything that comes handy, and then go off looking as grateful as if they had had oysters for dinner. Mules and donkeys and camels have appetites that anything will relieve temporarily, but nothing satisfy.
In Syria, once, at the head-waters of the Jordan, a camel took charge of my overcoat while the tents were being pitched, and examined it with a critical eye, all over, with as much interest as if he had an idea of getting one made like it; and then, after he was done figuring on it as an article of apparel, he began to contemplate it as an article of diet. He put his foot on it, and lifted one of the sleeves out with his teeth, and chewed and chewed at it, gradually taking it in, and all the while opening and closing his eyes in a kind of religious ecstasy, as if he had never tasted anything as good as an overcoat before, in his life. Then he smacked his lips once or twice, and reached after the other sleeve. Next he tried the velvet collar, and smiled a smile of such contentment that it was plain to see that he regarded that as the daintiest thing about an overcoat. The tails went next, along with some percussion caps and cough candy, and some fig-paste from Constantinople.
And then my newspaper correspondence dropped out, and he took a chance in that — manuscript letters written for the home papers. But he was treading on dangerous ground, now. He began to come across solid wisdom in those documents that was rather weighty on his stomach; and occasionally he would take a joke that would shake him up till it loosened his teeth; it was getting to be perilous times with him, but he held his grip with good courage and hopefully, till at last he began to stumble on statements that not even a camel could swallow with impunity. He began to gag and gasp, and his eyes to stand out, and his forelegs to spread, and in about a quarter of a minute he fell over as stiff as a carpenter’s work-bench, and died a death of indescribable agony. I went and pulled the manuscript out of his mouth, and found that the sensitive creature had choked to death on one of the mildest and gentlest statements of fact that I ever laid before a trusting public.
Twain wrote Roughing It in 1870–71, and it tells the story of his travels in the Wild West during the years 1861–1867, when he was 26–32 years old.