Question

why do solids expands a bit on heating and contract a bit on cooling

Answer 1

When heated, particles overcome intermolecular force of attraction and the solid starts expanding and on cooling the particles once again start coming near and so it contracts

Answer 2

solids expand on heating because the kinetic energy increase and the inter molecular space thus increase reducing inter molecular force of attraction. an don cooling the potential energy increases making extensive inter molecular force of attraction and less inter molecular space.

hope this helps!