Can't get your head around something 可以用来表示“搞不懂某事、不明白某事”。大家可以根据需要将表达中的 your 改成 my、our 等。
Roger has tried to explain the laws of cricket, but it sounds so complicated that I just can't get my head around it.
Everyone was laughing at Billy's joke but I couldn't get my head around it at all.
另一个和“头”相关的常用表达是 it's all in your head，意思是“全是凭空想象”。
I don't believe that your teacher wants you to fail the exams - it's all in your head!
Come on, don't be so superstitious. I'm sure the room is ok - it's all in your head!