“Affidavit” means a statement in writing of a fact or facts signed by the party making it, sworn to before an officer authorized to administer oaths, and officially certified by the officer under his seal of office. Tex. Gov’t Code Ann. § 312.011(1) (Vernon 1988). To constitute competent summary judgment evidence, affidavits must be made on personal knowledge, set forth facts as would be admissible in evidence, and show affirmatively that the affiant is competent to testify to matters stated therein. Tex. R. Civ. P. 166a(f).