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A lie-to-children, sometimes referred to as a Wittgenstein's ladder (see below), is an expression that describes the simplification of technical or difficult-to-understand material for consumption by children. The word "children" should not be taken literally, but as encompassing anyone in the process of learning about a given topic, regardless of age. It is itself a simplification of certain concepts in the philosophy of science.
Because life and its aspects can be extremely difficult to understand without experience, to present a full level of complexity to a student or child all at once can be overwhelming. Hence elementary explanations tend to be simple, concise, or simply "wrong" — but in a way that attempts to make the lesson more understandable. Sometimes the lesson can be qualified, for example by claiming "this isn't technically true, but it's easier to understand". In retrospect the first explanation may be easy to understand for its inaccuracies, but it will be replaced with a more sophisticated explanation which is closer to "the truth". This "tender introduction" concept is an important aspect of education.
Such statements are not usually intended as deceptions, and may, in fact, be true to a first approximation or within certain contexts. For example Newtonian mechanics, by modern standards, is factually incorrect, as it fails to take into account relativity or quantum mechanics, but it is still a valuable and valid approximation to those theories in many situations.
The term appeared in the book The Science of Discworld (2000), co-authored and partly based on ideas created by Terry Pratchett, and in The Collapse of Chaos (1994) and Figments of Reality (1997), both by the other two co-authors of The Science of Discworld, Ian Stewart and Jack Cohen.
The definition given in The Science of Discworld is as follows: "A lie-to-children is a statement that is false, but which nevertheless leads the child's mind towards a more accurate explanation, one that the child will only be able to appreciate if it has been primed with the lie". The authors acknowledge that some people might dispute the applicability of the term lie, while defending it on the grounds that "it is for the best possible reasons, but it is still a lie".
The reference to children seems at first to be literal, as the authors use different phrases for cases that involve adult audiences (for example they say of themselves "we belong to another, equally honorable profession: Liar-to-readers"). Later in the book, however, the term "lie-to-children" is used to include all examples of the pedagogical strategy, without reference to the age or composition of the audience. This inclusive usage can be understood as a metonymic extension of the literal sense, or as a self-demonstration, the early definition being a lie-to-children itself, implicitly refined to the "more accurate explanation" by its broader use later.
Wittgenstein's ladder 
My propositions serve as elucidations in the following way: anyone who understands me eventually recognizes them as nonsensical, when he has used them—as steps—to climb beyond them. He must, so to speak, throw away the ladder after he has climbed up it.
Related concepts and aspects 
The boundary is fuzzy between widespread misconceptions versus lies-to-children. One significant difference is that genuine misconceptions are resistant to further instruction, and are often believed firmly (sometimes passionately) by adults. On the other hand, students will easily recognize and discard the lies-to-children as more advanced concepts are acquired. Another significant difference is that a lie-to-children will tend to be an approximation of the truth (e.g., "You cannot find the square root of a negative number") while a misconception will often simply be wrong (e.g. tongue taste map, coriolis-in-the-bathtub).
The Buddhist version of Wittgenstein's ladder is Upaya (translated "expedient means"). Plato's version is The Noble Lie. While Upaya or the Noble Lie can be (as in Wittgenstein) teaching devices or stratagems to be superseded at a later stage, in many cases the laity only ever learns the exoteric doctrine, with only the elite ever learning the true esoteric version.