Pronouns have to agree in number with the words they refer to (called their antecedents). That is, a pronoun must be singular when its antecedent is singular, and plural when its antecedent is plural.

Isaac Newton – isn't he the guy who discovered gravity? Great! So, what's gravity? Aaaah, gravity is the force that makes objects fall towards the ground. And, without gravity, we'd all float! Hmmmm… ...