Take it into your head that…
Definition 1 of 1
Pro Tip 1/3Sudden Decisions
This phrase often implies a decision made quickly and without much thought. 
She took it into her head that she would dye her hair purple.
Pro Tip 2/3Questionable Reasoning
Use it when the reason behind someone's decision or thought seems unclear or odd. 
He took it into his head that he should start a farm in the desert.
Pro Tip 3/3Subjective Judgement
Reflects a subjective, personal decision, not necessarily shared by others. 
She just took it into her head that vintage clothes are the best.