How does your heart keep you alive? How does it pump blood? Why is blood so important? Why do children have heart surgeries? Why is a baby's heartbeat faster before it's born? Why does blood rush to your head when you're upside down? Why can you feel your heart in your head when you're lying still or under water? In this episode of But Why, we're going talking about a very special muscle! It keeps us alive and it has its own special rhythm: the heart. Pediatric oncologist Dr. Jane Crosson from Johns Hopkins Hospital answers questions about the heart.

Why do we laugh? Why do you feel ticklish when someone tickles you? Why can't you tickle yourself? We learn about how humor develops with Gina Mireault of the Infant Laughter Project at Northern Vermont University.

Why do people dream? Why do people have nightmares? How do dreams happen? Can people who are blind can see in their dreams? In this episode of But Why, we're answering dreamy questions with psychiatrist Dr. David Khan of Harvard Medical School. Download our learning guides: PDF | Google Slide | Transcript

Why do people need to sleep? How do we actually go to sleep? How does sleeping get rid of toxins in the brain? And how come when it's nighttime I don't want to go to sleep but when it's morning I don't want to wake up?! Those questions and more, all about sleep. We're joined by pediatric sleep psychologist Dr. Lisa Meltzer. Download our learning guides: PDF | Google Slide | Transcript

What are Olympic medals made of? Why does every country have a flag? The 2018 Winter Olympics are underway in PyeongChang, South Korea. We reached out to medal-winning Olympians Elana Meyers Taylor, Andrew Weibrecht and Hannah Kearney to reflect on what winning a medal represents. And we learn about flags with vexillologist Scot Guenter from San Jose State University.

How did the first fish get into the ocean? How do fish breathe under water? If you put a fish's head underwater, but not its tail, would it survive? How do fish get diseases? How do fish see underwater without googles? Why do fish swim when they are asleep? Do fish drink water? Do fish have ears? How are fish born? But Why visits the New England Aquarium in Boston to get answers to those and other questions kids have sent us about fish.

Instead of an episode of But Why, we're going to check out an episode of one of our other favorite podcasts. Circle Round is a storytelling show from WBUR, a public radio station in Boston. On Circle Round, they find stories from all around the world and then get really interesting people to act them out. This week we're sharing one of their episodes with you! This is one of our favorites. And it's actually about sharing. It's called 'The Lion's Whisker.'

In this episode, we answer a question from 5-year-old Wyatt in Los Angeles and learn about ancient underground cities in Turkey, the subterranean passageways of Montreal and the dug-out houses of Coober Pedy, Australia. Also in this episode: Why is it so warm underground?

We're marking the winter solstice with an episode all about snow! Why do snowboards look like skateboards? We get an answer from Burton Snowboards. How is snow made? Why is snow white? Why are all snowflakes different? We'll hear from Jon Nelson, author of "The Story of Snow: The Science of Winter's Wonder." Also why does snow melt? And where is the deepest snow?

In this episode of But Why we visit a credit union to learn what money is all about and Slate Money hosts Felix Salmon, Anna Szymanski and Jordan Weissman answer questions about why money plays such a big role in modern society. How was money invented? Why can't everything be free? How do you earn money? Why don't kids go to work? How was the penny invented? Why are dimes so small?