Today I found this in ubuntu forums,although it could be a long time ago I had to share it with you!

Easter Egg
Open terminal from Applications->Accessories->Terminal.Type these command in turns:

apt-get moo
aptitude moo
aptitude -v moo
aptitude -vv moo
aptitude -vvv moo
aptitude -vvvv moo
aptitude -vvvvv moo
aptitude -vvvvvv moo

Enjoy the output!If you have install apt-build(use sudo apt-get install apt-build),you could also type:

apt-build moo

cal 9 1752
We know,cal month year prints the calendar.But

cal 9 1752

Outputs:

September 1752
Su Mo Tu We Th Fr Sa
1 2 14 15 16
17 18 19 20 21 22 23
24 25 26 27 28 29 30

Here is the reason:

Britain and the British Empire (including the eastern part of what is now the United States) adopted the Gregorian calendar in 1752 (see the Calendar (New Style) Act 1750) by which time it was necessary to correct by 11 days. Wednesday, 2 September 1752 was followed by Thursday, 14 September 1752 to account for 29 February 1700 (Julian). After 1753, the British tax year in Britain continued to operate on the Julian calendar and began on 5 April, which was the “Old Style” new tax year of 25 March. A 12th skipped Julian leap day in 1800 changed its start to 6 April. It was not changed when a 13th Julian leap day was skipped in 1900, so the tax year in the United Kingdom still begins on 6 April.(from:http://en.wikipedia.org/wiki/Gregorian_calendar)

Next command is :

fortune

it may prints famous quotation,or something humourous and it prints different in each fortune command.
If you installed fortune-zh(sudo apt-get install fortune-zh),it can even prints Tang poem Song jambic verse .Of course,you’d first make your system support printing Chinese language.

Try other amusing commands:

yes
ddate
xeyes
cowsay (sudo apt-get install cowsay)
cowthink