When 24-year-old Spanish skateboarding star Danny Leon got made up to look like an 80-year-old man, his goal was o see if teens at a local skate park would teach him the sport.

They obliged. And when Danny started speeding down the half pipe and doing aerial spins, the kids were blown away.

Being a force of nature disguised as a harmless old guy is a pretty good metaphor for the way a blood clot can disguise itself as a simple bruise. But don't fall for the ruse.

Bruises can be painful and turn shades of black and blue, but generally they're not harmful. One caveat: Easy or spontaneous bruising can indicate underlying disease and a need to see your doctor.

A blood clot, on the other hand, is a concentrated aggregation of blood. It forms from an external injury to blood vessels or internal injury to the lining of a blood vessel from plaque, or because of dysfunction in your blood's flow-and-clot chemistry.

Clots can obstruct blood flow or dislodge and travel through your bloodstream, triggering heart attack, stroke, deep vein thrombosis (DVT), or pulmonary embolism (PE). So if you spot a clot, see your doc.

Here are some tips for spotting blood clots:

• Near your skin's surface, clots can appear bruise-like, but are generally redder and the underlying vein may be hard to the touch.

• A clot that's moved and is causing trouble may trigger swelling and pain in an extremity (DVT); slurred speech and vision problems (stroke); chest pain or upper body discomfort, shortness of breath, and a rapid heart rate (PE or heart attack).