A unilateral contract is one in which one party makes an express engagement or undertakes a performance, without receiving in return any express engagement or promise of performance from the other. Bilateral (or reciprocal) contracts are those by which the parties expressly enter into mutual engagements, such as sale or hire. Civ. Code La. art. 1758; Poth. Obi. 1, 1, 1. 2; Montpelier Seminary v. Smith, 69 Vt. 382, 38 Atl. 66; Laclede Const. Co. v. Tudor Ironworks, 169 Mo. 137, 09 S. W. 388.