It was a warm day. Warm enough for me to be wearing shorts and a t-shirt. Mum was wearing bright sundress that stopped about 5 inches above her knees. All day I had been catching glimpses down the front of it when she lent forward, as the top was low cut with a couple of thin shoulder straps. She had big creamy white breasts and her bra barely contained them.
We had finished the shopping and the heat off of the pavement was cruel as we made our way to the bus. We had to make a bit of run for it as it was already waiting at the stop. We managed to make it and I took the bags from mum while she paid for us. I wandered down the aisle to the only free seat. It was next to an old man who was sleeping. Mum followed behind me and stood in the aisle. I offered my seat to her but she politely declined it. We had been travelling for about five minutes when she changed her mind. Taking the bags from my lap, she plonked herself down and turned to me
"I hope you don't mind me sitting on your lap. I figure after all those years of sitting on mine, you owe me."
I didn't mind at all, even though the heat from her body was making me even warmer. The bumpy movements of the bus was causing mum's ass to rock against my crotch. I tried not to think about the sexual nature of it but I couldn't stop. I could feel the swell begin in my pants and I knew Mum would notice it any moment. She appeared to be concentrating on the passing scenery so I tried to wriggle myself out of this sticky situation. My movements only made things worse as I felt my erection slide right between the cheeks of her ass.
I'm not sure if Mum was uncomfortable with the situation or whether her dress was the problem, but she lifted her ass momentarily and flared her dress at the back. She settled back down, with my erection finding the groove of her ass straight away. Her movements had caused her dress to ride up at the front. My eyes were glued to her legs. They looked so inviting. I wanted to run my hands up them, but I knew it was inappropriate. I decided just to rest my hand midway along her bare thigh. I could get away with this innocently enough. Her skin was hot to the touch, but so smooth and soft. I nearly came right then. She didn't pull away which calmed my nerves a little. I slowly slid my hand down her leg until I reached her knee, then slid it back up. It was more affectionate than sexy, but I went a little higher than I had been previously. I repeated this a couple of times until my hand was at the top of her thigh. The hem of her dress was pulled right up to her panty line and I could feel the elastic of it through the cotton dress. Mum's eyes were closed but I could tell she wasn't sleeping. The movement of her ass on my crotch had changed from the random bounce of the bus to a regular back and forth motion. She knew what was going on and for some reason she wasn't stopping me. So I did what any horny young boy would do... I kept pushing my luck.