Why is it so hard for me to stop thinking?

I was trying to focus on my maths work for school, BUT My mind kept rambling. literally random stuff that might have happend to me earlier today. it doesnt have to be negative thoughts or thoughts linked to great emotion. its very annoying as i feel like i cant just fully lock in on my work because there is a conversation taking place at the same time at the back of my head. does anyone know anything about what i can do or if i might have something?