sink in
1. Lit. to sink, submerge, or descend into something. How long will it take the water to sink in? It might take days for the oil to sink in, so you have time to clean it up.
2. Fig. [for knowledge] to be understood. I heard what you said, but it took a while for it to sink in. I pay careful attention to everything I hear in calculus class, but it usually doesn't sink in.
sink something in (to) someone or something
and sink something in 1. Lit. to drive or push something into someone or something. The brave hero sank the wooden stake into the vampire. The hero sank in the stake.
2. Fig. to invest time or money in someone or something. (Sometimes implying that it was wasted.) You would not believe how much money I've sunk into that company! She sank in a lot of money, but it was all wasted.