The {n}th tetrahedral number is the sum of the first n Triangular numbers. Triangular numbers are the sum of the first n integers (1, 3, 6, 10, 15, etc.), and taking the sum of the triangular numbers results in the tetrahedral numbers (1, 4, 10, 20, 35, etc.)
The fifth tetrahedral number is the sum of the first five triangular numbers. The first five triangular numbers are 1, 3=1+2, 6=1+2+3, 10=1+2+3+4, and 15=1+2+3+4+5, so the sum of these numbers is 1+3+6+10+15= 35.
Derived from the meat counter at your local grocery. You're wating for your meat? Take a number and shut the heck up. Can now be used to shut someone up who is complaining about anything, or wanting to discuss something with you that perhaps you want to brush off.
"You want to talk to me about how I ran over your cat? Take a number"
"What, teacher? My homework is late? You want to discuss my grades? Take a number"
"you want to take me out to dinner? Dude, take a number"