(1) Be (talking) at cross purposes. = If two or more people are at cross purposes, they do not understand each other because they are talking different subjects without realising this.

• I think we’ve been talking at cross purposes- I meant next year, not this year.
• We’re talking at cross purposes.

(2) Keep oneself to oneself = to stay apart from other people ; to avoid other people.

• She was a shy girl who kept herself to herself.
• Titus is a very private person , he keeps himself to himself.

(3) Think big = to have plans to be very successful or powerful.

• You need to think big if you want to succeed.
• Dr. A.P.J.Abdul Kalam exhorted the school children to think big about our scientific growth and he said each Indian has the responsibility to safeguard the sovereignty the nation.