You have seen files named “resume.docx” or “photo.jpg” a thousand times, but those letters after the dot have a purpose. They’re called the file extension, and they help your computer identify the file type and choose a program to open it. “.docx” usually identifies a Word document. “.jpg” and “.png” identify image formats that store and compress pictures differently. “.pdf” identifies a document format designed to preserve its layout across devices—useful for sharing resumes and other documents. Here is the practical skill: if a file will not open, or opens looking wrong, check the extension before assuming something is broken. Renaming a photo to end in “.docx” will not turn it into a Word document. Changing the name does not convert the actual file format. Try this today: open your Downloads folder and look at what is in there. If your computer hides file extensions, turn that setting on. What is one file extension you have seen but never knew the meaning of? Turn on file extensions today, then drop the most unfamiliar one you find in the comments—or open Week 1 if you have not started yet.

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