Not just anyone is qualified to testify as an expert witness.  Edward Teller quoted Niels Bohr as describing an expert as "a person who has found out by his own painful experience all the mistakes that one can make in a very narrow field". *  The California Evidence Code takes a different approach as to who may be an expert witness:

(a) A person is qualified to testify as an expert if he has special knowledge, skill, experience, training, or education sufficient to qualify him as an expert on the subject to which his testimony relates.  Against the objection of a party, such special knowledge, skill, experience, training, or education must be shown before the witness may testify as an expert.

(b) A witness’ special knowledge, skill, experience, training, or education may be shown by any otherwise admissible evidence, including his own testimony.

NRS 50.275 similarly provides:

If scientific, technical or other specialized knowledge will assist the trier of fact to understand the evidence or to determine a fact in issue, a witness qualified as an expert by special knowledge, skill, experience, training or education may testify to matters within the scope of such knowledge.

These statutes called to mind a century-old criminal case in which the witness' testimony led to his own qualification:

Friend of mine was testifyin' for the defense in a case before Judge Stratton at Buckhorn once.  The lawyer for the defense was tryin' to qualify him as an expert witness, an' asked him how long he'd been a prospector.  "Thirty years," he says.  Well, pretty soon the the prosecutin' attorney gets to cross-examine him.

"How many years have you prospected?" he asks.

"'Bout, five years,' says the witness.

"Ah ha!" says the prosecutor, lookin' mean an' successful.  "Your Honor, I'd like to have you not that the witness under oath has already said he has been a prospector for thirty years, an' now he says he's prospected five year."

"Wait a minute, wait a minute," says the witness.  "I said I'd prospected five years, an' I have.  Both statements is facts, your Honor.  Five years I prospected and the other twenty-five I spent lookin' for my burrows.  You know how it is, your Honor."

"The witness qualifies as an expert, " says the j.p., thereby qualifyin' himself as a judge.

C.B. Glasscock, Gold in Them Hills 19-20 (1988).

Burros are not native to Nevada, but you will see many of them wandering the deserts (and towns) of Nevada.  Prospectors relied on burros to carry just about everything they needed.  Burros, however, have a tendency to wander and a large part of a prospector's time could be spent in tracking down his wayward "help".

A feral burro

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*R. Coughlan, Dr. Edward Teller's Magnificent Obsession, Life (Sept. 6, 1954) at 62.