我是个海绵只是瘦得不明显
[wŏ shì gè hăi mián zhĭ shì shòu dé bù míng xiăn]
Translates to 'I am a sponge; I just do not visibly look skinny.' It uses humor to convey that they have hidden depths (absorbs knowledge) or qualities, similar to a sponge storing water internally without showing it outwardly.