You know me. I'm a tiger when it comes to persuading, cajoling, begging you to go have some fun outside. Jump on your bike, walk in nature, paddle a canoe, do any thing you like that gets your heart pumping and your juices flowing, and keep your eyes, ears and fingers off your cellphone.

But this planet isn't getting any cooler, so it's time to remind yourself that exercising hard in extreme heat is a no-no. Heat illness is the No. 1 weather-related killer, according to the Centers for Disease Control and Prevention. Even if it doesn't kill you, exercising in high heat can make you very sick.

I've long known that intellectually, but I'd never seen it up close and personal. It happened very quickly, and it scared the sunlight out of me.

My friend was visiting me in Greece, and naturally, we headed for the beach. Her first mistake was to slather on the sunscreen and assume it protected her from the ill effects of the intense Mediterranean sun. It doesn't.

Next, she lay on the sand, hatless, her entire body exposed to the sun. Third, she was dehydrated and didn't know it. Finally, she was taking a heart medication and her doctor didn't warn her of possible complications from too much sun exposure. (What doctor does?)

Coming off the beach, we walked up a long hill to the house, and she collapsed in the kitchen. Luckily, she made it to a chair. She felt fireworks in her brain. Her vision blurred, her head buzzed, her arms tingled.

Most frightening? She was mentally disoriented. "I can't remember what I just thought," she said. We were on a remote island, a 10-hour ferry ride away from the nearest hospital. Sun stroke? A heart attack? We weren't sure what was happening.

Fortunately, we quickly got her cool and calm and hydrated, and after a few days of home care — more rest and fluids — and Skyping with her doctors, she felt much better and was able to take the boat to a plane and fly home. Heat exhaustion was the diagnosis. Her short-term memory is still a bit wobbly, but it's coming back, and Claire — a nurse! — feels she dodged a bullet and learned her lesson.

And so did I, which is why I decided to bow out of a long bike ride the other day when the temperature was 90-plus. Why risk it? Instead, I took myself inside for some yoga in a cool room with the fan blowing. Life is too short to pretend you're invincible.

And that's my plea to you, oh, active reader: Don't play around with exercising in high heat. Use your common sense. Don't overdo it. If you feel like walking, running, biking or just playing at the beach, do it in the early morning or late afternoon to avoid the sun's most intense rays.

No matter what time you're out, pay close attention to how you're feeling. If you're struggling to keep going, stop and cool off. Drink plenty of water. (But not too much, because that causes problems, too.)

And most importantly, learn the early warning signs of heat illness: including cramps, dizziness, headaches, nausea and thirst. If you experience any of these, don't push on. And remember what happened to Claire, with no early warning signs at all.

EAT THIS UP! GREENS FOR THE GRILL

Summer is everyone's favorite season for outdoor cooking, but don't think you're limited to steaks, brats, ribs and chops. Have you ever heard of grilled salad? You have now.

Romaine, endive, cabbage and radicchio are hearty lettuces that do well in high heat, if you don't let them cook too long. Separate the leaves, or cut the thicker species in half, brush lightly with oil and grill face down over medium heat.

Also yummy to grill: red peppers, zucchini, pineapple, watermelon (!!!) portobello mushrooms, artichokes (sliced through the middle) and — hold on now — avocado. Yes, grilled avocado. Makes the best guacamole in the taco-chip-eating world.

THREE WAYS TO BURN 100 CALORIES

1. Grocery shop for 20 minutes carrying a basket instead of pushing a cart.

2. Dance around the house for 20 minutes.

3. Mop the floors for 25 minutes.

ENERGY EXPRESS-O! TODAY'S BELLY LAUGH

"Self-delusion is pulling in your stomach when you step on the scale." — Paul Sweeney

Marilynn Preston — healthy lifestyle expert, well being coach and Emmy-winning producer — is the creator of Energy Express, the longest-running syndicated fitness column in the country. She has a website, marilynnpreston.com, and welcomes reader questions, which can be sent to [email protected]. She also produces EnExTV, a digital reincarnation of her award-winning TV series about sports, fitness and adventure, for kids of all ages, at youtube.com/EnExTV and facebook.com/EnExTV. To find out more about Preston and read features by other Creators Syndicate writers and cartoonists, visit the Creators Syndicate website at www.creators.com.