You get this:

<$let wikipedia="https://en.wikipedia.org/wiki/">
  <<wikipedia>>Aardvark
</$let>

This brings up a very important point about the order in which TiddlyWiki processes wikitext. When you transclude the value of a variable, or any of the other things that we'll be discussing in upcoming sections and chapters, including Procedures, Field Transclusions, and Macros, TiddlyWiki processes the wikitext within the variable separately from its surrounding context. You might expect that TiddlyWiki goes through the tiddler and expands all transclusions into one giant tiddler-long section of wikitext, and only then renders the wikitext to HTML. This is not the case!

Instead, it renders wikitext in discrete “chunks.” In this example, it's processing wikitext and it sees <<wikipedia>>. It consequently stops rendering the current chunk of wikitext (i.e., the whole tiddler), goes and renders the wikitext in the wikipedia variable, and then comes back and starts a new chunk. Therefore, when <<wikipedia>> is read, TiddlyWiki sees a separate chunk, which happens to be a link, and which it renders into a link. Then afterwards, it sees a chunk starting with the text Aardvark, and it renders that as text.