A popular math joke is:
 
Q: Why do mathematicians like parks?
A: Because of all the natural logs.
 
The natural logarithm (“natural logs”) joke has been cited in print since at least 2006. “Q: Why do mathematicians like forests? A: Natural logs!” was cited on Twitter on March 2, 2009.
   
     
Wikipedia: Natural logarithm
The natural logarithm of a number is its logarithm to the base of the mathematical constant e, where e is an irrational and transcendental approximately equal to 2.718281828459. The natural logarithm of x is generally written as ln x, loge x, or sometimes, if the base e is implicit, simply log x. Parentheses are sometimes added for clarity, giving ln(x), loge(x) or log(x). This is done in particular when the argument to the logarithm is not a single symbol, to prevent ambiguity.
   
Wikipedia: Mathematical joke (6 October 2006 page revision)
A mathematical joke is a form of professional humor which relies on aspects of mathematics or a stereotype of mathematicians to derive humor. The humor may come from a pun or double meaning of a mathematical term, or on a non-mathematician’s misunderstanding of a mathematical concept. Such jokes are frequently inaccessible to those without a mathematical bent.
 
Pun-based jokes
Why do Mathematicians like national parks?
Because of the natural logs.
   
mafiascum 
Alduskkel
Thu Feb 12, 2009 6:45 am
Why do mathematicians like parks? For all the natural logs!
 
Twitter
Hilary Mason
‏@hmason
I am WAY too amused by the Wikipedia ‘mathematical joke’ page: http://bit.ly/2PI3 Q: Why do mathematicians like forests? A: Natural logs!
9:03 AM - 2 Mar 2009
     
Studentawards Forum
Physics, math, engineering & comsci jokes
Superman6424
Posted: January 25, 2010 4:39:30 PM
(...)
Why do mathematicians like parks?
Because of the natural logs.
 
Twitter
Ms. Barbara Woodcock
‏@mswoodcockmath
Why do mathematicians like forests? Because they’re full of natural logs! #MathJokeMonday
6:47 AM - 4 Nov 2013
 
Twitter
Jake Thiessen
‏@TotesJake
@JessicaNaslund
Why do mathematicians like parks?
Because of all the natural logs
9:59 PM - 30 Apr 2014
 
Twitter
Holiday Inn Express
‏@hiegeorgetown
Have you ever wondered why mathematicians like parks?  I found out it was because of all the natural logs. 😊 #mathjokes
7:15 PM - 1 May 2014
 
Facebook
3D home tutorsㅿ
April 18, 2015 ·
Q: Why do mathematicians like parks?
A: Because of all the natural logs.
 
Facebook
Yaar Kahin Aap Love Guru Toh Nahi
May 22, 2015 ·
Q: Why do mathematicians like parks?
A: Because of all the natural logs
 
Twitter
Math Puns Daily
‏@fey_karl
Q: Why do mathematicians like parks?
A: Because of all the natural logs.
10:14 AM - 17 Jan 2016