The four rocky planets (Mercury, Venus, Earth and Mars) are much smaller than the four gaseous ones (Jupiter, Saturn, Uranus and Neptune). For example, the largest gas planet, Jupiter, is 318 times larger than the largest rocky planet, Earth. The explanation of this phenomenom is that the gaseous planets do not have a solid crust, therefore their materials are united in a less compact way, which amplifies their size.
I'd argue the nervous system because you sense pain through your nerves and when you realize something is poking you bam your nerves recognize it and tell your brain "UH OH CHEIF THERE'S A TACK" and you pull it out.
