You may have had an ultrasound before, and wondered what that gel does. Well, that weird alien goop has a purpose, and it has to do with being like our weird human skin.

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Sources:
General Ultrasound
http://teachersinstitute.yale.edu/curriculum/units/1983/7/83.07.05.x.html
http://courses.washington.edu/bioen508/Lecture6-US.pdf
http://www.physics.utoronto.ca/~jharlow/teaching/phy138_0708/lec04/ultrasoundx.htm
Ultrasound Gel
http://www.two-views.com/ultrasounds/gel-goo.html
https://healdove.com/health-care-industry/aquasonic_ultrasound_gel
Acoustic Impedance
http://www.schoolphysics.co.uk/age16-19/Sound/text/Acoustic_impedance/index.html

Images:
Sound Wave Diagram: https://commons.wikimedia.org/wiki/File:CPT-sound-physical-manifestation.svg