In 2012, we saw Venus transit the sun. It won't happen again until 2117, so if you missed it, you're our of luck when it comes to seeing that again. Today, you have a chance to see Mercury slide across the surface of our star.
It's cloudy here, so I'll be using NASA's special Mercury Transit website to show images of the planet as it crosses the sun during the day. If you want to learn more about the transit or what information will be displayed on the website, NASA has a good blog post explaining which regions they're focusing on and why we should care about observing transits like this. Also, be sure to check out the Solar Dynamics Observatory website from time to time for great images of the sun at various websites. They highlight solar phenomena (magnetic regions, sunspots, coronal mass ejections, etc) and really give our students a new view of our energy source. It's a powerful thing to show how complex our universe is.
Mercury repeats this trip thirteen times every hundred years (ish), so you'll have a chance to watch again in 2019 if you have to miss today.