diff --git a/factoids.toml b/factoids.toml
index c75b8e1..c17078d 100644
--- a/factoids.toml
+++ b/factoids.toml
@@ -7,3 +7,4 @@
 "!doesanyoneuse" = "Don't ask if anyone uses software xyz, just ask your specific question. This helps solve your problem quicker, and allows people who haven't used xyz but may still be able to help you to do so."
 "!library" = "Don't install libraries through nix-env or systemPackages. See https://nixos.wiki/wiki/FAQ#I_installed_a_library_but_my_compiler_is_not_finding_it._Why.3F for details."
 "!-A" = "You'll usually want to use nix-env -i with -A. It's faster and more precise. See https://nixos.wiki/wiki/FAQ#Why_not_use_nix-env_-i_foo.3F for details."
+"!tofu" = "To get a sha256 hash of a new source, you can use the Trust On First Use model: use probably-wrong hash (for example: 0000000000000000000000000000000000000000000000000000) then replace it with the correct hash Nix expected."
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