diff --git a/topics/04_mechanical/index.md b/topics/04_mechanical/index.md
index 07eb5de0f122636c44d30407276f3554f9943bc6..56c31c85e9c71c893a27d234cf00d6b9b991cc85 100644
--- a/topics/04_mechanical/index.md
+++ b/topics/04_mechanical/index.md
@@ -40,9 +40,9 @@ Non-contact Mode: Oscillates above the adsorbed fluid layer on the surface durin
 
 <a href="https://www.marposs.com/eng/application/technologies-touch-probes#:~:text=The%20working%20principle%20is%20quite,called%20the%20probe%20pre%2Dtravel.">Touch Probes</a>
 
-https://youtube.com/embed/vJdcbhGa7Jk
+<iframe width="560" height="315" src="https://www.youtube.com/embed/vJdcbhGa7Jk?si=YeFE7Buf8KPLohNa&amp;start=18" title="YouTube video player" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share" referrerpolicy="strict-origin-when-cross-origin" allowfullscreen></iframe>
 
-<p>resistance between the balls and rollers.</p>
+<p>Detecting contact by measuring resistance between the balls and rollers.</p>
 
 <img src="images/probe_light.png" alt="ProbeLight">
 
@@ -52,7 +52,8 @@ https://youtube.com/embed/vJdcbhGa7Jk
 <iframe src="https://youtube.com/embed/IcVv1So1n_8" frameborder="0" allowfullscreen=""></iframe>
 
 
-Optical Optically tracked touch probes
+Optical
+Optically tracked touch probes
 <a href="https://www.gom.com/nl/metrology-systems/gom-taster.html">GOM</a>
 
 <img src="images/gom_touch_probe.jpg" alt="GOM touch probe">
@@ -93,3 +94,5 @@ Add proliner
 ### Survey Prisms
 
 https://www.sumpoint.io/#services
+
+[Small Robots and Big Projects](https://papers.cumincad.org/data/works/att/acadia21_462.pdf), Maria Yablonina & James Coleman, ACADIA 2021