Left: tree 木, right: investigate carefully (originally two people 兄 discussing under a roof . Better in respect to 'examine' 験: a person 人 examining the horse's 馬 mouth 口 - [ are the mane and eyes 一 ])
Mnemonic
The tree with its covered opening is (by the person) investigated carefully.
Due to the use in “to examine 験”, this radical is seen as the head of a horse (with mane and eyes 一), in front of which a person stands 人 and checks/examines the mouth 口.
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