diff --git a/README.md b/README.md
index b9f6281..aead6f9 100644
--- a/README.md
+++ b/README.md
@@ -7,7 +7,15 @@ Analyze wikipedia dumps obtained from
 
 This project has two main parts, `sift` and `brood`.
 
-`sift` is written in Python and sifts through the wikipedia dump, parsing and
-analyzing individual articles and printing interesting data.
+### Sift
+
+`sift` is written in Python and sifts through a wikipedia article dump
+(`*-pages-articles.xml.bz2`), parsing and analyzing individual articles and
+printing interesting data.
+
+It takes a (decompressed) XML article dump on stdin. For each article in the
+dump, it prints a single-line JSON object to stdout.
+
+### Brood
 
 `brood` is written in Rust and analyzes the data obtained by `sift`.