For years I wrote most of the code for JHOVE. With each format, I wrote tests for whether a file is “well-formed” and “valid.” With most formats, I never knew exactly what these terms meant. They come from XML, where they have clear meanings. A well-formed XML file has correct syntax. Angle brackets and quote marks match. Closing tags match opening tags. A valid file is well-formed and follows its schema. A file can be well-formed but not valid, but it can’t be valid without being well-formed.
With most other formats, there’s no definition of these terms. JHOVE applies them anyway. (I wrote the code, but I didn’t design JHOVE’s architecture. Not my fault.) I approached them by treating “well-formed” as meaning syntactically correct, and “valid” as meaning semantically correct. Drawing the line wasn’t always easy. If a required date field is missing, is the file not well-formed or just not valid? What if the date is supposed to be in ISO 8601 format but isn’t? How much does it matter?
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According to the usual, somewhat simplified, description, a DNA molecule is a double helix, with its backbone made of phosphates and sugars, and four types of nucleotides forming the sequence. They are adenine, guanine, thymine, and cytosine, or A, G, T, and C for short. They’re always found in pairs connecting the two strands of the helix. Adenine and thymine connect together, as do guanine and cytosine.
It’s been too long since I’ve had a special discount on FTL. For all of June, you can get 
Fact-checking the GIF format
The Politifact article on the White House’s video “evidence” against reporter Jim Acosta looked plausible enough to me, until I got to the explanation of GIF files. It got significant points wrong, following common misunderstandings.
The regular readers of this blog mostly know what GIF really is, but this article may be a useful reference if you need to explain to anyone. The Politifact article says:
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Tagged GIF