Borrowing warmth instead of making it
Why Do Lizards Sit in the Sun?
A lizard has no way to warm itself from the inside. Everything it does that day depends on finding the right rock.
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Step 1 of 5
Cold muscle is slow muscle.
The chemistry that makes a muscle pull runs faster when it is warm. A cold lizard is genuinely sluggish — it cannot run from anything until it has warmed up.
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Step 2 of 5
So the morning is spent warming.
A lizard comes out at first light and lies flat side-on to the sun, flattening itself to catch as much light as possible on the largest area.
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Step 3 of 5
Then it spends the day dodging heat.
Once warm it does the opposite. Too hot is as dangerous as too cold, so the lizard shuttles between sun and shade all day, holding itself in a narrow band.
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Step 4 of 5
Not making heat is cheap.
Staying warm from the inside costs a huge amount of food. A lizard eats a small fraction of what a mammal its size needs, which is why deserts suit lizards and not mice.
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Step 5 of 5
The tail is a way out.
Many lizards can drop their tail at a weak point built into the bone. It thrashes on the ground, holds the attacker's attention, and the lizard leaves without it.
A regrown tail is not the same. The new one is cartilage rather than bone, and it usually comes back a different colour and shape.
The short version
A lizard makes no heat of its own, so it shuttles between sun and shade all day to hold itself at a working temperature.
Try it yourself
Put one hand on a sunny windowsill and one on a shaded floor. That difference is the lizard's whole thermostat.