Please explain why the following may not be valid contracts:

1. Fred has suffered from dementia for many years. Some days he is quite lucid and other days he is not. He recently signed a contract to purchase a jet ski.

2. Mary wrote out a contract for her neighbor's child to mow her lawn every week until school started. The boy gladly signed the contract. The boy's mother witnessed the signing.

3. Joan paid her friend Jason to do "donuts" with his motorcycle in her neighbor's front yard. Joan and the neighbor had been fighting for years and vandalizing each other's property.

4. Alicia was a frustrated paralegal who was tired of the very slow copy machine in her office. One day a salesman came in and had a wonderful one day deal on a new copy machine. The attorney was in court that day, so she signed the contract for the new machine on his behalf believing the attorney would be quite pleased.

5. Laura owns several quarter horses and always worries about having enough hay to feed them during the winter months. She found a farmer who would sell her hay and entered a contract with him for hay to be delivered sometime during the growing season at a price to be determined at the time of the delivery.