Alexander (P) sued Kramer Bros. Freight Lines, Inc. (D) for injuries that occurred when P's and D's trucks collided at six o'clock in the morning when there was spotty fog on the Turnpike. Both P and his truck were seriously injured. D denied its negligence and raised the defense of contributory negligence. P claimed that D’s truck had suddenly pulled in front of him. D claimed that P had been following him too close for a longer period of time and had eventually hit him from behind. The two drivers' stories at trial were very different as to the location and manner of the accident. In a conference with the judge, D said that he accepted the court's statement that 'the burden of proof is on the defense for contributory negligence,' but D did not request a jury charge for contributory negligence nor did he object to the charge that was actually given as was required under Rule 51. When the judge gave the instructions to the jury on the issue of who bears the burden of proof for contributory negligence, it was wrong. Normally the burden of proof for contributory negligence is on the plaintiff to prove that he was not contributorily negligent. The jury found for P and D appealed; the court's jury charge for the burden of proof on contributory negligence was in error, and the error was reversible error. Rule 51 requires any objections to jury charges to be made clearly before the jury retires. Except in rare circumstances, a failure to object means a waiver of any right to appeal on the charge. Note: the burden is on the plaintiff for proving freedom from contributory negligence.