A problem is only believable if it makes sense with the character's situation.
Therefore, if your character is brushing his teeth in the story and you introduce the problem of his brother getting captured by aliens, that doesn't really fit the story well, and isn't very believable.
If you happen to have your character brushing his teeth, when the toothbrush accidentally slips and cuts the inside of his mouth, that's much more believable!
Write down a good example from a novel you've read in literature circles this year in your spirals.
Choose a mundane situation that your character is in and have them experience a believable problem. Come up with several more problems that you can imagine your character experiencing during this mundane situation.
Choose one of those problems above to expand the moment. Start by showing the reader the mundane situation, and gradually introduce the problem.
"Students will create believable problems for their characters and begin developing those problems into stories."