Which side of the 2-in-1 lid should I use for hot drinks like coffee or tea?

The 2-in-1 lid design on your Terracotta Orange 30oz tumbler gives you options, but for hot beverages—coffee, tea, hot chocolate—the slider sip opening is the optimal choice for several important reasons. First, the direct sip opening allows you to control the flow rate by tilting the tumbler, letting you take small, manageable sips of hot liquid without overwhelming your mouth. Second, the sip opening enables you to enjoy the aroma of your coffee or tea, which is a significant part of the hot beverage experience that's lost with straw drinking. Third, and most importantly, using a straw for very hot liquids can be dangerous—the liquid flows directly into your mouth without the temperature-testing opportunity that sipping provides, increasing burn risk. The slider mechanism on the sip side gives you a moderate-sized opening (about 1cm x 3cm) that's large enough for comfortable drinking but small enough to prevent spills or excessive flow. When you're done sipping, simply slide it closed—the seal is immediate and secure. The stainless steel around the opening doesn't get uncomfortably hot even with boiling water inside thanks to the vacuum insulation keeping the exterior cool to touch. For cold drinks—water, iced coffee, smoothies—the flip straw side is often more convenient, especially during activities where you can't or don't want to tilt the tumbler (driving, working out, typing at a desk).