system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do downloads often stop at 99% before finishing?
gpt
Three possible reasons:

**1. File integrity verification:** Your computer performs an operation on the data you downloaded and compares the result to a checksum. If answer and checksum do not match, the data is corrupt. This is how your computer tells if your file's data is a completely correct copy. 

**2. Uncompressing:** Some optimised downloaders or protocols (usually modern ones) have built-in compression. Compression reduces file size and helps with download duration. However, to extract the raw file from a compressed one, it needs to be decompressed first. The 1% is decompressing: you may notice the 1% takes longer for bigger files. 

**3. Virus scan:** Modern browsers also have in-built virus scanning. The process is similar to file integrity verification, but your computer actively looks and matches for malicious code checksums.