Why do sidewalks expand in hot weather?

Question

by the way i am in grade 7 so write stuff that i can understand

Answer 1

The concrete that composes the sidewalk absorbs the solar radiation (Sun's rays) that hits it.  This absorption results in the sidewalk being heated, and heating of an object normally results in expansion of that object.

Answer 2

To make a long story short, high temperatures make objects expand  and cold weather makes objects get smaller (like the mercury in a thermometer).