When we exercise, it's that much more important that we drink enough water. But hold on a second - just how much constitutes enough? For one, a simple rule to go by is that if you're thirsty, you've already waited too long. Your initial signs of thirst are a relatively benign indicator that you're already slightly dehydrated. However, in case you're not really into simple rules, Women's Health magazine published a handy article that spells out just about how much water you lose (and, therefore, how much you will need to replace) for a variety of physical activities.

1 1/2 hours of Singles Tennis: 25 ounces of water lost

35 minutes in a Spinning Class: 35 ounces of water lost

2 Hour Bike Ride: 57 ounces of water lost

Skiing for 7 hours: 89.6 ounces of water lost

4 hour and 20 minute Marathon Run: 117 ounces of water lost

8 hour Hike with a Backpack: 135 ounces of water lost

Cleaning the House for 1 hour: 4 ounces of water lost

Strength Training for 30 minutes: 6.8 ounces of water lost

Swimming for 45 minutes: 10 ounces of water lost

5-Mile Road Run: 22 ounces of water lost