'Show, don't tell' is a guideline. It absolutely DOES NOT mean that every single thing must be shown not told. That would be dreadfully wearying for the reader.
For example, let's look at someone starting to cry. Let's say this occurs in a courtroom drama, where the character on the witness stand on trial for murder unexpectedly breaks down and confesses. The 'starting to cry' part of that incident might be the first sign of what's coming, and should be shown, not told. 'Jenny stopped speaking mid-sentence and stared at the back wall of the courtroom. You could've heard a pin drop. She swallowed, and tears welled in her eyes. As they started to spill down her cheeks, she whispered, "I did it."'
On the other hand, let's say that it occurs in an argument between a 13-year-old girl and her mom over a boy the girl wants to date. Every single reader knows that the girl is going to break out in tears any moment now, and when it happens between two lines of dialogue, it can just be 'Jenny started to cry,' and you carry on. Showing it (as in the example above) would slow down the dialogue and interfere with the pace of the argument.
In addition, the complementary guideline that goes alongside 'show, don't tell' is 'scene and summary'. Most work alternates between scene (show) and summary (tell). If you analyse published works, you'll see this over and over. The 'summary' bits give the reader a bit of a mental break before moving into the next 'scene' bit.
One example of using summary effectively is in a series, where certain key events from previous books need to be quickly summarised so that readers who didn't read them understand what happened. You don't 'show' the event because you already did in an earlier book, you simply summarise it. Read a Harry Potter book that's not the first one, and you'll see that JKR is a master of this technique.
The trick to effective writing is to understand all the tools at your disposal and know which one to employ at any given time. That, of course, is much easier said than done.
