Why does 八 have an extra thing on top?

Question

The similarity between "to enter (入)" and "number 8 (八)" confuses me. What's the extra part on top of 八?

Answer 1

Not all fonts have that horizontal thing. Here are examples of the same 八 rendered with various fonts installed on my PC:

Basically, that horizontal bar is something similar to serifs in Latin fonts. When you write kanji by hand, you should not try to reproduce it. When in doubt, find a 教科書体 font (blue).

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Answer 2

It is a design typically used for the typeface called Minchotai (roughly equivalent to Latin serif fonts). Basically it is designed so that glyphs are closer to those written with brushes.

In typography it seems to be called 八屋根 (hachi yane), literally "eight roof". Quoting the image and text from here:

右下の6文字が「八屋根」と呼ばれる要素で、「八」や「入」などの右払いの入りに付く横画です。

Translation: The 6 characters on the bottom right have the element called "hachi yane", which represents the starting of "migi barai" strokes (strokes going diagonally from left to right) as seen in 八 and 入.