思's top isn't really a field — it began as 囟, the fontanel, the crown of a baby's skull. Skull above, heart 心 below: the ancient Chinese theory that thinking happens between head and heart, written as one character.
意思 'meaning' powers the untranslatable 不好意思 (embarrassed / excuse me) and 有意思 (interesting). 思念 is missing someone. Li Bai's 静夜思 'Quiet Night Thoughts' — moonlight mistaken for frost, head lowered, thinking of home — is the first poem every Chinese child learns.
Skull on top, heart 心 below — THINKING with both.
小意思 'small meaning' is what you say handing over a gift — 'it's nothing'. 意思意思 doubled means a token gesture, often a red envelope. Meaning, in Chinese, is negotiable currency.