Triangular number

Imagine you have a bunch of blocks and you want to stack them up in a particular way. First, you take one block and stack it in a line. This is the first triangular number. Then, you take two blocks and stack them in a line next to the first block, but you also stack one block on top of the first block and two blocks on top of the second block. This is the second triangular number. Next, you take three blocks and stack them in a line next to the first two blocks, but you also stack one block on top of the first block, two blocks on top of the second block, and three blocks on top of the third block. This is the third triangular number.

The pattern continues until you have a stack of blocks that looks like a triangle - hence the name "triangular number." To find the nth triangular number, you simply add up all the numbers from 1 to n. For example, the fifth triangular number would be 1 + 2 + 3 + 4 + 5 = 15.

Triangular numbers are useful in many different areas of math and science, including geometry and number theory. They also have some interesting properties - for example, every odd number is the difference between two consecutive triangular numbers.