"Yuki, you don't mind if I use your computer later, do you?" These words, more or less, are what you hear your mother whenever you're about to leave for school ever since you taught her how to use it recently. You have no qualms about her using it. What you are concerned about is what exactly she does on it.
You have no way of knowing what she does there since she only uses it while you're away. By the time you're home, she'd be doing her usual chores. It seems like she figured how to not leave a trace of her actions on it. You're sure that she's doing something inappropriate, but then again, you don't really know. For all you know, she could be viewing cat pictures. If only there was an opportunity to see what she was doing on your computer...
Luckily for you, such an opportunity arises.
Due to a timely incident at school, you had to leave early. Usually you'd notify your mother about it—but not this time. Quickly, you run out of class and back to your house. You arrive home and enter quietly, slowly creeping to your room. As you near your room, you begin to hear a faint moan coming from the door. Mere inches from the door, you lightly twist the knob and open the door slightly ajar—enough for it to not squeak—and begin to peek.
You see your mother—seated with one leg bent and rested on your chair—as she touches herself to something on the computer. The moans were definitely hers.
"What the..." you mutter under your breath, dumbfounded by this sight, as you feel a stirring in your pants.
As you continue to peep on her,you accidentally lean too much onto the door, causing it to open up and squeak. This catches your mother's attention, and quickly turns her head over to you.
"Y-Yuki!" she exclaims as she pulls her skirt up in a hurry and frantically covers the monitor with her hands.
"M-mom! What the...?!" you exclaim, feigning surprise.
The two of you briefly stare at each other in silence, before she begins to slowly look down to the erection tenting your pants.