The zig-zag surface of air filters can trip walking robots, but they also provide small "steps" for climbing.  LEGO robot building instructions at <a rel="nofollow" class="external free" href="https://www.diywalkers.com/strider-builds.html">https://www.diywalkers.com/strider-builds.html</a>
<a rel="nofollow" class="external text" href="https://www.youtube.com/channel/UCpucV3Q9cE_PsVFcyyPDuKw">DIY Walkers</a>
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<bdo lang="en" dir="ltr"><a href="//commons.wikimedia.org/wiki/Commons:YouTube_files" title="Commons:YouTube files">YouTube</a>: <a rel="nofollow" class="external text" href="https://www.youtube.com/watch?v=asztws6--U8"><bdi lang="">Strider Robot vs. Air Filter Ramp (medium motors)</bdi></a> – View/save archived versions on <a rel="nofollow" class="external text" href="https://web.archive.org/web/*/https://www.youtube.com/watch?v=asztws6--U8">archive.org</a> and <a rel="nofollow" class="external text" href="https://archive.today/https://www.youtube.com/watch?v=asztws6--U8">archive.today</a></bdo>