1 to exchange

2 exchange

3 communication

4 interaction

5 to have social contact (with sb)

1 to be connected

2 traffic

3 transportation

4 communications

5 liaison

1 sky

1 starry sky

2 the heavens

1 time and place

2 world of a particular locale and era

3 (physics) space-time

1 lit. doesn't (even) enter a single aperture (of one's head)

2 I don't understand a word (idiom)

3 it's all Greek to me

1 a worthless piece of paper (idiom)

1 grasp this fundamental point and all the rest will follow (idiom)

1 to act in collusion (idiom)

1 mutual exchange of assistance (idiom)

2 to benefit from each other's strengths and make up each other's shortfalls

3 to reciprocate with material assistance

4 scratch each other's back