The Legally Blonde star, 44, shared footage of a home learning session as they promoted Kiwico's monthly project boxes for kids.
But their adorable mother-son interaction and funny faux pas completely stole the show.
WATCH: Reese Witherspoon and her son Tennessee have hilarious home-learning lesson
In the video, Reese and Tennessee attempted to make a sloth teddy but when he pointed out some of the string had fallen into her drink they both broke into fits of laughter.
"Is this brown yarn supposed to be in your tea?" Tennessee asked his famous mother, who then dragged out the long piece of wet fabric from her cup.
When they finally did get to the end of their project, Reese asked for a high-five, only to find Tennessee left her hanging.
Reese loves being a mum
Reese shares her youngest child with her husband, Jim Toth, but she also has Ava, 21, and Deacon, 17, from her first marriage to Ryan Phillippe.
The star opened up about motherhood and what it means to her when she appeared on The Drew Barrymore show recently.
"You take the food out of your mouth, the clothes off your back, the sleep out of your life," she added.
"But every bit of it - every bit of that sacrifice is truly worth it because you feel like, that's what makes me wake up on a Sunday. It's not movies or my job, it's my kids."
She also admitted that becoming a mum so young, and also trying to launch her career was difficult.
Reese recently welcomed a new addition to her family - her pet dog
"I got pregnant when I was 22 years old," she said. "And I didn't know how to balance work and motherhood. You just do it, you know?
"I'd made movies but I hadn't established myself as someone who could demand that it shoot close to my kids' school or, you know, I didn't have any real power, leverage within my industry.
"I was just like every other mum trying to figure it out - and dad out there, and partner, and grandparent who's raising a child."
Looks like she's done a great job!
Read more HELLO! US stories here
Like this story? Sign up to our newsletter to get other stories like this delivered straight to your inbox.