ring expansion

yes nishant bhaiya is bang on

if u want to satisfy urself then look up bayer strain theory....ull get the details

Examples of S_Ni ring formation...rings can be formed other ways too.
Ring formation is governed by two factors : kinetic and thermodynamic stability. The kinetic factor is based on orientation, for example if a nucleophile and leaving group are present on a chain anti-periplanar(orientation) to each other, the nucleophile knocks out the leaving group and a ring forms. This is the case with three membered rings, they are formed quickly and can break quickly as well.
Three membered rings are favoured kinetically and not thermodynamically.
Four membered rings have considerable angle strain as well, and are not favoured either way too well. Orientation as well as stability are unavailable.
Five membered rings lack the orientation somewhat to form, as anti-periplanarity may be present but the distance in the chain is greater. However, thermodynamically, it is quite stable and is hence formed.
Six membered rings, though, have EXCEPTIONAL thermodynamic stability and form irrespective of unavailable orientation.
Taking the specific case of cyclohexane, it has two main conformations, chair and boat. Groups present(H as well) are labelled equitorial and axial. If I remember correctly, interactions between groups are minimized as 6 hydrogens are in equitorial positions and 6 are in axial positions. Steric hindrance in this conformation is minimum while there is some angle strain.
However in the boat conformer, heavy steric hindrance is present(flagpole interaction between axial groups) and even though angle strain is absent, this causes it to be quite unstable. Thus cyclohexane is present mostly in the more-stable-than-cyclopentane chair conformer.
Jitna yaad tha utna bata diya :D