diff --git a/core/hci/hci-summary.pdf b/core/hci/hci-summary.pdf
index 25141e3..161ac80 100644
Binary files a/core/hci/hci-summary.pdf and b/core/hci/hci-summary.pdf differ
diff --git a/core/hci/parts/01_ucd/00_intro.tex b/core/hci/parts/01_ucd/00_intro.tex
index 59cf087..ee3f5bb 100644
--- a/core/hci/parts/01_ucd/00_intro.tex
+++ b/core/hci/parts/01_ucd/00_intro.tex
@@ -13,4 +13,16 @@ So, what is it then? It is characterized by:
     \item \bi{Empirical measurement} {\scriptsize (Come up with some sort of measurement of performance)}
     \item \bi{Iterative Design} {\scriptsize (Identify and fix problems)}
 \end{enumerate}
-That all is very obvious and
+That all is very obvious and the hardest part for Computer Scientists of course is the first property.
+The second property is typically done via user studies, where you keep track of a lot of data on how users interact with your application.
+Don't shy away from storing way more details than you think you could possibly use, it will make it much easier to evaluate things later on.
+    {\tiny (Can you tell that I am speaking from experience?)}
+
+That brings us to a formal definition from ``The Design of Everyday Things'' by Donald A. Norman (2013),
+which also includes famous examples such as the \hlhref{https://www.youtube.com/watch?v=qtCEoGyfsxk}{Norman Door}.
+
+\begin{definition}[]{User-Centred Design}
+    User-Centered Design (UCD) is an iterative design process in which designers focus on the users and their needs in each phase of the design process.
+    UCD involves researching user behaviors, needs, and experiences and incorporating user feedback through multiple iterations of prototyping and testing to ensure usability,
+    accessibility, and satisfaction. The goal is to develop solutions that align with real user needs rather than assumptions about those needs.
+\end{definition}