The stabilizing effect of alkyl substituents on carbocations can be explained through

Hyperconjugation

Explanation:

Hyperconjugation is interaction of electrons in σ-bond with nearby empty of partial filled p orbital or pi- orbital to create expanded molecular orbital which increases the stability of compound.  

Alkyl substitute like methyl and ethyl are electron donating group and thus stabilizes carbocation because the electrons around the neighboring carbons are attracted to neighboring positive charge and thus reduces the deficiency of electron in positively charged carbon

float.

archimedes' principle

denser than water sinks in water.

denser than mercury would sink in it